BDU, IOT By Abeyou Wale April 11-20/2011 Bahir Dar Ethiopia

Transcription

1 Regional Training Course on Open-Source GIS and RS for IWRM Watershed Caharaterzation using Open-Source GIS BDU, IOT By Abeyou Wale April 11-20/2011 Bahir Dar Ethiopia Abeyou Wale. 1

2 Contents 1. Introduction Relevance of DEM in Hydrology DEM hydro-processing Step-by-step Procedures Downloading SRTM Importing SRTM in ILWIS and changing the coordinate system Transform from Lat / Long to a metric projection Hydrological DEM processing some pre processing steps Fill Sinks: Flow Direction: Flow Accumulation: Drainage Network Extraction: Drainage Network Ordering: Catchment Extraction: Catchment Merge: Definition...29 Abeyou Wale. Abeyou_wale@yahoo.com 2

3 1. Introduction In order to perform distributed or even lumped rainfall-runoff modelling a multitude of information is needed. Part of the necessary model input can now be provided through processing and analysis of a Digital Elevation Model (DEM) in combination with information extracted from other remotely sensed images of a selected model area. One of the most recent near global elevation data sets is the one recorded during the 11 day Shuttle Radar Topographic Mission based on a C-band interferometer radar configuration. This information, representing the radar reflective surface (which may be vegetation, man-made features or bare earth!), was collected in February 11, 2000 and is now available at a horizontal spatial resolution of 3 arc second approximately 90m resolution (averaged from 30 by 30 metres) for over 80% of the globe, excluding north and south poles. The SRTM swaths extended from about 30 degrees off-nadir to about 58 degrees off-nadir from an altitude of 233 km, and thus were about 225 km wide. This data is currently distributed free of charge by USGS can be downloaded freely from unzipped, mosaiced and processed. As for arid and semi-arid areas the reflective surface mostly represents the actual ground surface, the main limitations of the dataset is that there are often small areas having data voids which need to be corrected prior to further processing. A Digital Elevation Model (DEM) is a digital representation of ground surface topography or terrain. It is also widely known as a Digital Terrain Model (DTM). A DEM can be represented as a raster (a grid of squares) or as a triangular irregular network. DEMs are used often in geographic information systems, and are the most common basis for digitally-produced relief maps. This document will discuss the methods to be followed to extract the watershed characteristics such as area, perimeter, drainage density, longest flow path length etc of a catchment using open source GIS from SRTM DEM using open-source GIS software called ILWIS Relevance of DEM in Hydrology DEMs are used in water resource projects to identify drainage related features such as watershed divides, valley bottoms, channel networks and surface drainage patterns, and to quantify subcatchment and channel properties such as size, length, slopes etc. The DEMs have proved to be very efficient in extracting the characteristics of a watershed for modeling purposes. DEMs are potentially valuable tool for the topographic parameterization of hydrological models especially for drainage analysis, hill slope hydrology, watersheds, groundwater flow and contaminant transport etc. The accuracy of these topographic parameters is a function of both the quality and resolution of the DEM used in the extraction. Digital Elevation Models have a very wide range of applications. They form one of most frequently used spatial data sources in GIS projects. They are also the basis for a large number of derivative information. The most important application areas of DEMs are: Slope steepness maps, showing the steepness of slopes in degrees, percentages, or radians for each location (pixel). Slope direction maps (also called slope aspect maps), showing the orientation or compass direction of slopes (between ). Slope convexity/concavity maps, showing the change of slope angles within a short distance. From these maps you can see if slopes are straight, concave or convex in form. Hill shading maps (or shadow maps), showing the terrain under an artificial illumination, with bright sides and shadows. Three dimensional views showing a bird s eye view of the terrain from a user defined position above the terrain. Abeyou Wale. Abeyou_wale@yahoo.com 3

4 Cross-sections indicating the altitude of the terrain along a line and represented in a graph (distance against altitude). Volume maps (or cut-and-fill maps), generated by overlaying two DEMs from different periods. This allows you to quantify the changes in elevation that took place as a result of slope flattening, road construction, landslides etc DEM hydro-processing in ILWIS The open source GIS Integrated Land and Water Information System (ILWIS) will be used for the training; the software can be freely downloaded from 52 North Initiative for Geospatial Open Source Software ILWIS features include digitizing, editing, analysis and display of data as well as production of quality maps. ILWIS was initially developed and distributed by ITC Enschede (International Institute for Geo-Information Science and Earth Observation) in the Netherlands for use by its researchers and students, but since 1 July 2007 it has been distributed under the terms of the GNU General Public License and is thus free software. The DEM hydro-processing tool of ILWIS is able to extract the drainage network and the catchments based on outlet location and stream order, the module has the following procedures: Fill sink : used to clean up single or multiple depressions artificially from the DEM Flow direction: determines into which neighbouring pixel any water in a central pixel will flow naturally. Flow accumulation: performs a cumulative count of the number of pixels that naturally drain into outlets. Drainage network extraction: extracts a basic drainage network (boolean raster map). Drainage network ordering: examines all drainage lines in the drainage network map, finds the nodes where two or more streams meet, and assigns a unique ID to each stream in between these nodes, as well as to the streams that only have a single node. Catchment extraction: constructs catchments; a catchment will be calculated for each stream found in the output map of the Drainage network ordering operation. Catchment merge: is able to merge adjacent catchments, as found by the Catchment extraction operation. The detail description of input and output maps of the DEM Hydro-process operations are discussed below: FILL SINK OPERATION Before using the Flow Direction operation, you may wish to clean up your DEM from local single or multiple pits, so that local depressions (sinks) are removed from your DEM to ensure hydrological connectivity. The Fill sinks operation will 'remove' the following from a Digital Elevation Model Depressions that consist of a single pixel, i.e. any pixel with a smaller height value than all of its 8 neighbouring pixels, Depressions that consist of multiple pixels, i.e. any group of adjacent pixels where the pixels that have smaller height values than all pixels that surround such a depression. Process: When a depression of a single pixel is encountered: then the height value of this pixel will be increased to the smallest value of its 8 neighbour pixels. When a depression of multiple pixels is encountered: then the height values of this depression will be increased to the smallest value of a pixel that is both adjacent to the outlet for the depression, and that would discharge into the initial Abeyou Wale. Abeyou_wale@yahoo.com 4

5 depression. Some depressions are natural (e.g. in a limestone area) or the catchment area consists of a closed basin. In this case these areas have to be assigned undefined and are therefore not considered during the fill pit operation. Figure is adopted from ILWIS help file FLOW DIRECTION In a (sink-free) Digital Elevation Model (DEM), the Flow direction operation determines into which neighbouring pixel any water in a central pixel will flow naturally. Flow direction is calculated for every central pixel of input blocks of 3 by 3 pixels, each time comparing the value of the central pixel with the value of its 8 neighbours. The output map contains flow directions as N (to the North), NE (to the North East), etc. You can choose whether you wish to calculate the flow direction for the central pixels: by steepest slope: find the steepest downhill slope of a central pixel to one of its 8 neighbour pixels, or by lowest height: simply find the neighbour pixel that has the smallest value of all 8 neighbours, while this value should also be smaller than the value of the central pixel. When the position of the steepest-slope-neighbour pixel or the lowest-heightneighbour pixel is determined, the flow direction for the central pixel is known. The lowest height computation method determines the elevation difference between the cell and a neighbour cell only but the steepest slope computation takes in to account cell distance as 1 for neighbouring cells in N-S or W-E direction and 2 for cells in NE-SW or NW-SE directions. Figure is adopted from ILWIS help file FLOW ACCUMULATION The Flow accumulation operation performs a cumulative count of the number of pixels that naturally drain into the outlets. The operation can be used to find the drainage pattern of a terrain. As input the operation uses the output map of the Flow Direction operation. The output map contains cumulative hydrologic flow values that represent the number of input pixels which Abeyou Wale. Abeyou_wale@yahoo.com 5

6 contribute any water to any outlets (or sinks if these have not been removed); the outlets of the largest streams, rivers etc. will have the largest values. Figure is adopted from ILWIS help file DRAINAGE NETWORK EXTRACTION The Drainage Network Extraction operation extracts a basic drainage network (Boolean raster map). The output raster map will show the basic drainage as pixels with value True, while other pixels have value False. As input: the output raster map of the Flow Accumulation operation is required. This map contains a cumulative drainage count for each pixel. When a stream threshold value of 1000 is used: In the flow accumulation map if flow accumulation is greater than 1000, this pixel will be assigned value True in the output drainage network map; else value False will be assigned to the output pixel. Figure below show a drainage network for 1000, 1500 and 5000 threshold number The Drainage network extraction operation extracts the basic drainage network (Boolean raster map). DRAINAGE NETWORK ORDERING The Drainage network ordering operation: Examines all drainage lines in the drainage network map, uses output map from the Drainage network extraction operation and finds the nodes where two or more streams Abeyou Wale. Abeyou_wale@yahoo.com 6

7 meet, then the procedure will assign a unique ID to each stream in between these nodes, as well as to the streams that only have a single node. The output of this operation is a raster map, a segment map and an attribute table that all use a newly created ID domain. The attribute table contains information on each stream, such as: Strahler ordering number, Shreve ordering number, Stream length, calculated along the drainage, and calculated as a straight line between XY coordinates, Slope values in degrees and in percentages, calculated along the drainage and calculated as a straight line between XY-coordinates, and elevation, Sinuosity of the drainage path as a measure of meandering, Total upstream drainage length, i.e. the total length of the streams that drain into the current one, etc. Principles of Strahler and Shreve network ordering: In the attribute table, a Strahler column and a Shreve column will be found. These columns contain values that reflect the position of a stream between its adjacent upstream and down streams. The ordering systems have a different manner of calculation. First, the ordering will start form the upper-most starting points of the drainages in the network. These streams obtain ordering number 1 (both in the Strahler and in the Shreve ordering system), until a node is found that connects the stream with a following stream (down-flow). For next streams down-flow, Strahler ordering numbers are calculated as follows: When two (or more) streams of equal order join each other, the stream order value is increased by 1. For example, when two streams with order number 2 join each other, the next stream will receive order number 3. When a higher-order stream joins a lower-order stream, the order number for the next stream does not increase; instead, the largest order number of the streams that contribute to it is assigned. For example, when a stream with order number 1 joins a stream with order number 2, the next stream will also be assigned order number 2. For next streams down-flow, Shreve ordering numbers are calculated as: The sum of the Shreve ordering numbers of the streams that directly contributes to this stream. For example, when a stream with order number 1 joins a stream with order number 2, the next stream will be assigned order number 3. Below, you will find two pictures explaining the Strahler and Shreve ordering systems. Abeyou Wale. Abeyou_wale@yahoo.com 7

8 Example of drainage network ordering map CATCHMENT EXTRACTION The catchment extraction operation constructs catchments; a catchment will be calculated for each stream found in the output map of the drainage network ordering operation. The operation uses a Flow direction map to determine the flow path of each stream. As input: The output raster map of the drainage network ordering operation and; The output raster map of the flow direction operations is required. As output a raster map, a polygon map and an attribute table are produced which all use the ID domain of the input drainage network ordering map. The attribute table contains information on each catchment, such as: Area and perimeter of the catchment, Total upstream area, i.e. the area of all catchments that drain into this catchment, etc. Abeyou Wale. Abeyou_wale@yahoo.com 8

9 CATCHMENT MERGE The catchment merge operation is able to merge adjacent catchments, as found by the Catchment extraction operation. In fact, new catchments will be created on the basis of the Drainage network ordering map and its attribute table. As input: The output map and table of a previous Drainage network ordering operation, The output map of a previous Flow direction operation and; The output map of previous Flow accumulation operations is required. You can merge catchments in two manners: By specifying a point map that contains locations of stream outlets within a catchment; all adjacent catchments that drain into such outlets will be merged, By simply specifying a Strahler or Shreve ordering value: all adjacent catchments that have this Strahler or Shreve order value (or a lower value) and which drain into a common catchment will be merged. As output a new catchment raster map, polygon map and attribute table are produced. These all use a new ID domain. The attribute table contains information on the new catchments, similar to the output attribute table of the Catchment Extraction operation, but you will also find information on: Total drainage length, total upstream area, Drainage density, Longest flow path length and longest drainage length. Abeyou Wale. Abeyou_wale@yahoo.com 9

10 1.3. General process used to obtain relevant hydrological information using DEM Hydro-Processing of ILWIS Abeyou Wale. 10

11 2. Step-by-step Procedures 2.1. Downloading SRTM Before you start with the exercises, download a digital elevation model at web site of the selected area see figure below. Product : Data File Name : srtm_44_10.zip Latitude min: 10N max: 15N Longitude min: 35E max 40E Center point : Latitude N Longitude E Download (FTP) file NOTE: For this exercise zipped SRTM DEM of the Lake Tana Basin with a file name srtm_44_10.zip is given in the working folder under D:\TaSBO\Data\Day4 directory, Unzip srtm_44_10.zip by right clicking and choosing Extract to srtm_44_10\. The file will be extracted under the folder name srtm_44_10, from now on we will make use of this folder. You can have a quick look on the contents of this folder; there are four different files. Abeyou Wale. Abeyou_wale@yahoo.com 11

12 readme.txt: is the metadata which explains the data format, precision and datum. srtm_44_10.hdr: High Dynamic Range (HDR) image format; used for enhancing the color and brightness range of a digital image. srtm_44_10.tfw: this is used to georeference a *.tif file. The *.tfw file provides real world coordinate information that allows a corresponding.tif file to be correctly positioned on a map or in a mapping system. srtm_44_10.tif: Tagged Image File Format (abbreviated TIFF) is a file format for storing images 2.2. Starting ILWIS Go to desktop and double click the ILWIS icon, then navigate to the working folder where the unzipped digital elevation model srtm_44_10 is located D:\TaSBO\Data\Day4\srtm_44_10 using the navigation panel on the operations tree see the figure below. Abeyou Wale. Abeyou_wale@yahoo.com 12

13 2.3. Importing SRTM in ILWIS and changing the coordinate system Importing the DEM of *.tif file format to the ILWIS environment Procedure Go to File menu Import > Map Choose the import format as Tagged Image File Format from the list box Choose Srtm_44_10.tif from the left text box Edit the output name as Lake_DEM. See figure below, then click on Ok button at the lower right corner, wait for some time the file will be imported. The file is imported under srtm_44_10 folder under ILWIS catalogue Coordinate system: World Geodetic System (WGS) is a standard for use in cartography, geodesy, and navigation. It comprises a standard coordinate frame for the Earth Domain: which shows the possible values of a map (this can be value, id, class ) Georeference: explains the extent of the map and pixel size. Coordinates of lower left corner and upper right corner Raster map: is a set of grids with elevation information. Make a right click and check the corner coordinates of Lake_DEM raster ( ) map on the properties command. As you can see the coordinate system is a geographic latitude longitude coordinate system. Which is approximately lower left of 10N, 35E and upper write 15N, 40E Abeyou Wale. Abeyou_wale@yahoo.com 13

14 Opening the raster file Double click on the raster file Lake_DEM, accept the default display option setting and click on Ok button, now you will be able to see the digital elevation model. Left click on the map and move you curser, those values displayed on the screen are the elevation information. Did you found Lake Tana in this map? How? Double click on Lake_DEM-map list menu of the ILWIS sub-window (as shown in the red box) and change the representation to CLRSTP10 then click on Ok button, as shown in the figure below. Now you can see the lake and the flood planes around Fogera and Dembia. Are you able to see Lake Tana watershed divide? How? 2.4. Transform from Lat / Long to a cartisian coordiante system (projection) The imported Lake_DEM is in latitude longitude coordinate system that means this is measured along the earth curvature. Now we have to project this map to a Cartesian coordinate x,y system. To project the latlon coordinate system, follow the next procedure: Create a new coordinate system at the ILWIS main window, coordinate system projection and use the parameters below to define this projection (see figure 3). File / create / coordinate system Make the coordinate system as Lake_Metric and check the radio button for coordinate system projection since the DEM has already a coordiante system as shown below, finally click on Ok button Then click on Projection button: Abeyou Wale. Abeyou_wale@yahoo.com 14

15 Click on the projection and choose for the projection, ellipsoid and datum from the list as shown below. Projection parameters To resample the imported DEM to the new metric system of Lake_metric coordinate system, First transfer corner coordinates of the latlon coordinates to a metric coordinate system using transfer coordinates on the operation list Inter minimum latitude and longitude values and maximum latitude and longitude values on the input coordinate as shown below Under the operation list double click on as shown in the above write side figure and inter the lower left and upper rite corners of the geographic coordinate and choose the new coordinate system lake_metric under output coordinate system list box, this will give you the new values of the bounding box in cartesian coordinate system. Abeyou Wale. Abeyou_wale@yahoo.com 15

16 Keep the out put values to create the georeference of the metric system These are the bounding box for the metric system Lower left corner , Upper right corner , Right click on Lake_DEM raster map / image processing / resample... Use a nearest neighbour resampling method, Out put name as Res_Lake, create a new georeference by clicking on located at the left of Georeference list box with the same name as the output map(res_lake), using the coordinate system defined above (Lake_metric), above pixel size of 90 and inter corner coordinates of the raster map (out put of the transfer coordinates) and keep others as default and click on Ok button; Click ok and show, the software will do the resampling within minutes wait for some moment when it finish click on Ok button, move you cursor what do you observe? Close the newly vreated Res_Lake raster map and make a right click on the newly created Res_lake raster map and choose properties This is the bounding box of the Res_Lake in the newly created Coordiante system, previously the Coordiante system along the earth curvature. Abeyou Wale. Abeyou_wale@yahoo.com 16

17 2.5. Hydrological DEM processing some pre processing steps The hydrologic modeling functions in ILWIS DEM Hydro-Processing analyst provide methods for describing the physical components of a surface. The hydrologic tools allow you to identify sinks, determine flow direction, calculate flow accumulation, drainage network extraction, drainage network ordering, delineate watersheds, and create stream networks. Deriving runoff characteristics such as upstream elevation, downstream elevation, strahler and Shreve order, drainage length, slope angle of drainage, straight length of drainage and sinuosity of the catchment, then using different formulas we can estimate time of concentration, lag time, time to peak and peak discharge. This session will focus on DEM parameter extraction, especially those parameters relevant for hydrological analysis. For this purpose use will be made of the module called DEM Hydroprocessing and can be found in the ILWIS operation tree. Creating a submap To make the DEM processing fast let as make a submap, is just to focus on smaller area Open the resampled map Res_Lake then click on file menu / submap click on the Coordiante radio button and write the coordinates and output file name as shown below, then click on ok button followed by show. To start the hydrologic processing the tools as given in the right hand figure are available in the ILWIS Operation-tree. The following is a short discussion on the Flow Determination functionality Fill Sinks: Before using the Flow Direction operation, you should clean up the Digital Elevation Model (DEM), so that local depressions (sinks) are removed from your DEM. Double click Fill Sinks command under flow determination operation of DEM hydro-processing Abeyou Wale. Abeyou_wale@yahoo.com 17

18 Click on show button In order to evaluate the location of the sinks and the elevation differences at those locations, the map produced using the fill sink operation can be subtracted from the original DEM using mapcalc; Write the following on the command line: Sink:= Fill_sinks Lake_submap and press enter Where: Sink: the output map created Fill_sinks: map produced using fill operation Lake_submap: input elevation map As you can see from the output map, there are a number of local depressions especially at the central eastern part of the submap there are areas which are filled up to 110 meters Flow Direction: The Flow direction operation determines into which neighbouring pixel any water in a central pixel will flow naturally. Abeyou Wale. Abeyou_wale@yahoo.com 18

19 Result of flow direction operation: click left mouse button and move around the map the output map contains flow directions as N (to the North), NE (to thenorth East), etc Flow Accumulation: The Flow accumulation operation performs a cumulative count of the number of pixels that naturally drain into outlets. Abeyou Wale. Abeyou_wale@yahoo.com 19

20 Result of flow accumulation operation: move the curser pointer over the map and simultaneously clicking the left mouse button. The values represent the number of upstream pixels contributing to the target pixel. In other words, if multiplied by the pixel area the value represents the upstream catchment area in raster format Drainage Network Extraction: Flow accumulation map can also be used to extract the drainage network in raster format. A minimum contributing catchment area can be assumed which is needed to generate a first order stream. In the flow accumulation map this can be represented by a threshold, if contributing area is larger than the threshold the pixel can be assigned drainage, else it is assumed to be overland flow. Smaller threshold number increase the drainage network ordering processing time needed and the drainage density will be high. Click on Network and Catchment Extraction followed by double clicking on Drainage Network Extraction Result of drainage network extraction: the red one is with 1000 stream threshold, the green one with 2000 stream threshold numbers and the blue one with 4000 stream threshold number for Ribb and Guamra rivers. Abeyou Wale. Abeyou_wale@yahoo.com 20

21 Drainage Network Ordering: The Drainage network ordering operation: examines all drainage lines in the drainage network map, finds the nodes where two or more streams meet, and assigns a unique ID to each stream in between these nodes, as well as to the streams that only have a single node. The input DEM should be your Lake_submap (and not the filled DEM) as this map is used to extract elevation information that is used as an attribute in the associated topology tables that are generated as well. The option Minimum Drainage Length allows for removal of very short drainage lines. You can set this threshold to 800 metres, therefore drainage lines shorter than this threshold will be removed. Specify appropriate output map for your final extracted drainage (both a vector and a raster map are created). The output of this operation is a raster map, a segment map and an attribute table that all use a newly created ID domain with the same name Drain_net_order. Depending on the size of the input maps and the initial drainage threshold selected this operation might take a while. Display the resulting raster map using the default representation. Abeyou Wale. Abeyou_wale@yahoo.com 21

22 The output attribute table will contain the following columns: Upstream The IDs of the streams that directly contribute to the current stream, e.g. when LinkID streams 5 and 7 flow together into stream 12, then the UpstreamLinkID column will read for the record with ID 12: {5,7} UpstreamCoord The XY-coordinate of the beginning of a stream segment (down-flow); UpstreamElevation The elevation, as extracted from the DEM, at the position of the upstream coordinate. This column is a value column. DownstreamCoord The XY-coordinate of the end of a stream segment (down-flow). DownstreamElevat A column containing the elevation, as extracted from the DEM, at the position ion of the downstream coordinate. Elevation Height difference between upstream coordinate and downstream coordinate, Difference i.e. UpstreamElevation - DownstreamElevation. Strahler Strahler ordering number; Shreve Shreve ordering number; the sum of the Shreve order values of the UpstreamLinkIDs. Length StraightLength SlopeAlongDraina ge% SlopeDrainageStra ight% Sinuosity TotalUpstreamAlo ngdrainagelength StrahlerClass The length of a stream measured along the drainage. The length of a stream calculated as the difference between the upstream coordinate and the downstream coordinate, i.e. UpstreamCoord - DownstreamCoord The average slope in percentages between the upstream coordinate and the downstream coordinate, when the length is measured along the drainage, i.e. (ElevationDifference/Length)*100% The average slope in percentages between the upstream coordinate and the downstream coordinate, when the length is the straight length between the upstream coordinate and the downstream coordinate, i.e. (ElevationDifference/StraightLength)*100% A measure for meandering of the stream, i.e. Length / StraightLength The total length of all (upstream) streams that contribute to the current stream An additional class column to be able to display the Strahler ordering numbers as classes. Strahler and shreve ordering respectively Abeyou Wale. Abeyou_wale@yahoo.com 22

23 Catchment Extraction: For each drainage segment created the corresponding catchment area is computed. This is again a raster map. Also here an attribute table is computed giving a number of relevant variables (such as upper stream and downstream link, perimeter, area etc of the catchments). The output of this operation is a raster map, a segment map and an attribute table that all use a newly created ID domain with the same name as Catch_Extr. Columns in the Catchment extraction output attribute table: domain DrainageID UpstreamLink Catchment The IDs of the table's domain, every record (ID) represents a certain catchment that corresponds to a certain stream. The ID of the stream that corresponds to this catchment; the ID of the table and this column always show the same number. This column is a value column. The IDs of the catchments that directly contribute to the current catchment, e.g. when catchments 5 and 7 flow together into catchment Abeyou Wale. Abeyou_wale@yahoo.com 23

24 18, then the UpstreamLinkCatchment column will read for the record with ID 18: {5,7} DownstreamLink The ID of the catchment into which the current catchment will flow Catchment (down-flow). This column is a value column. Perimeter The perimeter of the current catchment. CatchmentArea The area of the current catchment. TotalUpstreamArea The total area of the catchments that directly contribute to the current catchment, i.e. the sum of the areas of the catchments listed in column UpstreamLinkCatchment. LongestFlowLength The length of the longest flow path found in this catchment, according to the Flow direction input map. CenterCatchment The XY-coordinate at the center of each catchment; these coordinates are shown as points in the fourth picture above. This column is a coordinate column. CenterDrainage The XY-coordinate in the middle of each stream segment; these coordinates are shown as points in the third picture above. This column is a coordinate column Catchment Merge: These single catchments have to be merged as there are far too many. In order to do so a merging can be done using Strahler or Shreve order, but also using one or multiple user defined outlet locations. Within this operation also the drainage can be extracted for the selected subcatchment area and the longest flow path segment can be computed. Catchment merging with defined outlet location For this case we will try to extract the catchment of Ribb and Gumara rivers with the given outlet location. The outlet location is the location of the staff gauging station installed by ministry of water resources; the location is collected by hand held GPS don t trust this outlet location for further analysis. River X Y Ribb Gumara To create a point map for the outlet location: Open drain_net_order rater map Go to File / create / point map write the Outlet on the Create Point dialogue box and click on Ok button Abeyou Wale. Abeyou_wale@yahoo.com 24

25 Adding Ribb and Gumara outlet point x,y location Go to Edit menu /add point and enter the X and Y coordinates of Ribb and Gumara rivers outlet respectively as shown below step by step Finally to extract the catchments of Ribb and Gumara rivers you have to check whether the outlet is located on the drainage network order or not. If it located away from the drainage network order it has to be edited to be at the top of the drainage network order. For the Ribb case the outlet Coordiante will overlay on the drainage network order but for the Gumara case the outlet will be a bit far away from the drainage network, so the outlet has to be moved or edited to place on top the network using the move point toolbar. After modifying the outlet locations, double click on the catchment merge operation under Network and Catchment Extraction Abeyou Wale. Abeyou_wale@yahoo.com 25

26 As output a new catchment raster map, polygon map and attribute table are produced. These all use a new ID domain with the same name as Ribb_Gumara. Abeyou Wale. Abeyou_wale@yahoo.com 26

27 Columns in the Catchment merge output attribute table: DrainageID UpstreamLink Catchment DownstreamLink Catchment Perimeter CatchmentArea TotalUpstreamArea TotalDrainage Length DrainageDensity (m/km2) LongestFlowPath A column listing the IDs of all streams located within a new catchment. The ID(s) of the new catchments that directly contribute to this new catchment, e.g. when catchments 1, 2, 3, and 4 flow together into catchment 5, then the UpstreamLinkCatchment column will read for the record with ID 5: {1, 2, 3, 4} The ID of the new catchment into which a current new catchment will flow (down-flow), e.g. when catchment 5 flows into catchment 6, then the DownstreamLinkCatchment column will read for the record with ID 5: 6. This column is a value column. The perimeter of each new catchment. The area (m 2 ) of each new catchment. The total area (m 2 ) of the catchments that directly contribute to a current catchment, i.e. the sum of the areas of the catchments listed in column UpstreamLinkCatchment. The sum of the lengths of all drainages in a catchment. The drainage density within a catchment as TotalDrainageLength / CatchmentArea The length of the longest flow path found in a catchment, from the Abeyou Wale. Abeyou_wale@yahoo.com 27

28 Length catchment's outlet to the most distant source on the catchment boundary, according to the Flow direction and Flow accumulation input maps. LongestDrainage The length of the longest actual stream within this catchment. Length CenterDrainage The XY-coordinate in the middle of a longest flow path. This column is a coordinate column. CenterCatchment The XY-coordinate at the center of a catchment. This column is a coordinate column. Output of catchment merge operation with catchment boundary, longest flow path and drainage network. The point map locates location of stream outlet. Observe the contents of Ribb_Gumara attribute table, what do you observe? Abeyou Wale. Abeyou_wale@yahoo.com 28

29 3. Definition Ellipsoid: For many maps it is assumed that the Earth is a sphere. However, because of the Earth s rotation, the shape of the Earth is not a perfect sphere. Actually, the Earth is slightly flattened towards the poles: the equatorial axis (line from the center to the equator) is longer than the polar axis. The Earth's shape can better be represented by an ellipsoid. Over the years, various different ellipsoids are calculated. Variations in calculated ellipsoids are due to the irregularities in the surface of the Earth. The choice of the ellipsoid which fits best a certain region of the Earth surface to be mapped, depends on the surface curvature and undulations in that region. Hence every country has its own 'best fit' ellipsoid. Projection: Projections are designed to solve the problem of drawing objects which are located in a spherical coordinate system (on the earth's surface) in a planar coordinate system (XY-coordinate system), i.e. on a piece of paper. The problem can be compared by making a whole orange, halve an orange peel or part of an orange peel appear flat. Datum: A geodetic datum defines a reference ellipsoid for a particular region, oriented to the landscape, with an 'initial point' of reference on the surface. The 'initial point' is assigned a latitude, longitude, an elevation above the ellipsoid, a direction of the vertical, and the azimuth of a point in the vicinity (to fix the North). Once a datum is adopted, features on the ground in a given area can be mapped relative to the adopted ellipsoid and the adopted 'initial point'. Coordinate system: A coordinate system contains information on the kind of coordinates you are using in your maps; you may for instance use user-defined coordinates, coordinates defined by a national standard or coordinates of a certain UTM zone. A coordinate system defines the possible XY- or LatLon-coordinates that can be used in your maps. Universal Transverse Mercator System UTM: is a cylindrical projection, meaning the globe is encircled by an imaginary cylinder touching at the equator, and the earth is projected onto the cylinder. The Mercator projection is a conformal projection, meaning that angles and small shapes on the globe project as the same angles or shapes on the map. The price paid by all conformal projections is great variation in scale away from the central portions of the map. Greenland on a Mercator map looks as big as South America, though it has actually only 1/8 the area. Abeyou Wale. Abeyou_wale@yahoo.com 29