GEOG4017 Geographical Information Systems Lab 8 Spatial Analysis and Digital Terrain Modeling

Transcription

1 DEPARTMENT OF GEOGRAPHY HONG KONG BAPTIST UNIVERSITY Prof. Q. Zhou GEOG4017 Geographical Information Systems Lab 8 Spatial Analysis and Digital Terrain Modeling Objectives The exercise is designed to familiarize students with the creation and use of digital terrain model (DTM) and raster (grid) datasets. Students are asked to perform an investigation on landslide risks in Tai Lam Chung area using terrain and other geographical data. During the exercise, students will practice techniques and procedures to derive geomorphic parameters such as slope and aspect from digital elevation data, and manipulate them with other raster data using Spatial Analyst and 3D Analyst. Tasks include: Create DTM by using contour lines and spot heights; Derive Slope and Aspect from the DTM; Extract information from raster values by using Raster Calculator and Reclass; Guidelines Preparation 1. Logon to workstation using your GISLAB account. Check the contents of K:\geog4017\lab8. The folder contains two shapefiles and four grids. t6_elev.shp contour lines of the region (20meters interval) (elevation in meter are stored in the HEIGHT attribute) t6_height.shp Spot heights of the region (elevation in meter are stored in the HEIGHT attribute) t6cult A raster dataset shows the distribution of cultivated land (VALUE = 1) in this region t6path A raster dataset shows the area within 20m of footpath (VALUE = 1) and paved trail (VALUE = 2) in this region. t6res_tlc A raster dataset shows the extent of Tai Lam Chung Reservoir (VALUE = 1). 2. Start ArcMap with a Blank Map. In Catalog window copy the K:\geog4017\lab8 folder to your J:\geog4017 For this exercise, we need to use 3D Analyst and Spatial Analyst extensions for various analyses. Select Customize > Extensions. Tick the 3D Analyst and Spatial Analyst if they are not ticked, and click [Close] to close the dialog box. Then save your map document (File > Save) as lab8.mxd and place it in J:\geog4017\lab8.

2 The ArcGIS Spatial Analyst provides a broad range of powerful spatial modeling and analysis features. You can create, query, map, and analyze cell based raster data; perform integrated raster/vector analysis; derive new information from existing data; query information across multiple data layers; and fully integrate cell based raster data with traditional vector data sources. Creation and conversion of DTM 3. Before we start our tasks, we need to create a DTM. ArcGIS can create and store three types of surface model: raster, TIN, and terrain. Here we will create the DTM in raster. We can do it by creating a TIN using the contour lines and spot heights, then convert the TIN into raster. search What is a TIN surface? in ArcGIS Desktop Help Firstly add the feature classes t6_elev.shp and t6_height.shp (both in J:\geog4017\lab8\) into the data frame as layers. Open and check their attribute tables. You may find that the height of contour lines and spot heights are stored in the HEIGHT field (attribute) of the tables. Close the tables. Launch the ArcToolBox by selecting Geoprocessing > ArcToolbox (if it is not launched). From the 3D Analyst Tools toolbox, Data Management > TIN toolset, open the Create TIN tool. In the Create TIN dialog box, set the parameters as below: Output TIN: J:\geog4017\lab8\t6_tin Coordinate System (optional): (click the button, then select Hong_Kong_1980 Grid from Layers)

3 Input Feature Class (optional): Input Features Height Field SF Type Tag Field t6_elev HEIGHT Hard_Line <none> t6_height HEIGHT Mass_Points <none> (untick) Constrained Delaunay (optional) (click the cell to change the Height Field and SF Type if necessary) Click [OK] to create the TIN. Once the TIN is created, it will be added to your data frame as a layer automatically. 4. Next we will convert the DTM from TIN to Raster. Switch to the ArcToolBox. From the 3D Analyst Tools toolbox, Conversion > From TIN toolset, open the TIN to Raster tool. Set the Input TIN as t6_tin, name the Output Raster to be t6dtm (place it in J:\geog4017\lab8). In Sampling Distance (Optional) field, select CELLSZIE (value may be varies), change to value to 10. Leave other fields as defaults. Press [OK] to create the raster. Change to value to Check the contents of the TIN t6_tin and Raster t6dtm.

4 Question: What are the minimum and maximum elevation (in meter) in this region? Minimum: Maximum: The layers t6_height, t6_elev and t6_tin may not be used in our further steps. You may remove it from the data frame by right clicking it then select Remove. Solving real life spatial problem using Raster Calculator and Reclass 6. In this task, you have been engaged as a consultant to investigate the risk of landslide in T6 region (extent please refer to sheet #6 of HM20C series map) Hong Kong. You are requested to produce a statistical table which shows the distribution of landslide risk in this region. Landslide Risk Risk Scoring Legend Area (m 2 ) % of total area No Risk 0 White Very Low Green Low Blue Medium Brown Severe Red Excluded area or area without data Black Total Area (m 2 ) The risk of landslide is influenced by three factors: Slope (LU), Aspect (AS), Path and Trail (PT). The weighting for these factors are accumulative (Total Risk Scoring = LU + AS + PT). Slope (LU) Weighting 0 <5 0 5 < or above +60 Aspect (AS) Weighting Flat 0 Southeast +10 Southwest, South, East +15 North East, West +20 North, Northwest +25

5 Path and Trail (PT) Weighting Within 20m of footpath (FPI) +40 Within 20m of paved trail (TAL) +20 Moreover, Tai Lam Chung Reservoir and all cultivated lands will be excluded from your investigation. We will use the DTM and other relevant data (as listed in step 1) to do our works. 8. Add the grid t6cult, t6path and t6res_tlc (all in J:\geog4017\lab8\) into the data frame as layers. When you try to add the grids into data frame, a dialog box will appear to ask whether you want to build a pyramid for each grid. Select [No] here. search Raster pyramids in ArcGIS Desktop Help 9. Now you should have FOUR layers in your data frame (t6dtm, t6cult, t6path, t6tlc_res). Check their properties (right click > Properties ) as well as their attribute tables (right click > Open Attribute Table). Questions: What are the cell size of all grids (raster) here? The cell size is defined when you create the grid. Where did you define the cell size during the creation of t6dtm? How many values are there in each grid and what is/are its/their meaning? t6cult t6path t6tlc_res t6dtm 10. DTM may be used to derive relief data such as slope, aspect, curvature, etc. In our case here we need the slope (LU) and aspect (AS) data. Derive slope from the t6dtm grid. Switch to the ArcToolBox. From the 3D Analyst Tools toolbox, Raster Surface toolset, open the Slope tool. Select t6dtm as Input raster and set J:\geog4017\lab8\t6slope as Output raster, set Output measurement (optional) to be Degree, leave Z factor (optional) as default. Press [OK] to create the

6 grid. 11. Derive aspect as well from the t6dtm. Switch to the ArcToolBox. From the 3D Analyst Tools toolbox, Raster Surface toolset, open the Aspect tool. Select t6dtm_grid as Input raster and set J:\geog4017\lab8\t6aspect as Output raster. Press [OK] to create the grid. Question: Why ArcGIS does not use value 0 to represent flat area in t6aspect? 12. We already have the data for all three factors (LU, AS, PT). Next we will reclassify the values in t6slope, t6aspect and t6path as to represent the risk caused by these factors respectively. We will use reclassification of slope as an example. Switch to the ArcToolBox. From the 3D Analyst Tools toolbox, Raster Reclass toolset, open the Reclassify tool. Set Input raster as t6slope and Reclass field as VALUE (the only choice). Delete all existing entries (except > ) in Reclassification by selecting the entries and press [Delete Entries] button. Then press the [Add Entry] button to add entries as below. Old values New values

7 Name the Output raster to be t6slope_rc (place it in J:\geog4017\lab8). Press [OK] to create the grid. 13. Do the same jobs on t6aspect to create a grid that shows the risk of each cell that caused by aspect. Reclassify it as below: Old values New values Set the Output raster to be t6aspect_rc (place it in J:\geog4017\lab8). Click [OK] to create the grid. 14. Do the same jobs on t6path to create a grid that shows the risk of each cell that caused by path and trail. This time set the Reclass field as TYPE and reclassify it as below: Old values FPI 40 TAL 20 0 New values

8 Set the Output raster to be t6path_rc (place it in J:\geog4017\lab8). Click [OK] to create the grid. Question: Can you guess why this time we need to reclassify into 0? 15. Next we will combine the risk factors as to obtain the final risk scoring by adding the t6slope_rc, t6saspect_rc and t6path_rc together. We will use the Raster Calculator to perform the calculation. Switch to the ArcToolBox. From the Spatial Analyst Tools toolbox, Map Algebra toolset, open the Raster Calculator tool. search How Raster Calculator works in ArcGIS Desktop Help Add the three grids together by typing the following formula in the Raster Calculator: "t6slope_rc" + "t6aspect_rc" + "t6path_rc" Set the Output raster to be t6risk (place it in J:\geog4017\lab8). Click [OK] to create the grid. Questions: What are the expected minimum and maximum values in t6risk? Minimum: Maximum: What are the real minimum and maximum values in your t6risk? Minimum: Maximum:

9 Can you point out the difference between the value 0 and in this grid? search in raster datasets in ArcGIS Desktop Help 16. We have already gotten the risk scoring of different area in this region. However, remember that Tai Lam Chung Reservoir and all cultivated land should not be included in our study. Those areas are listed in t6res_tlc and t6cult grids. We may use the same tools (Reclass and Raster Calculator) to exclude them in our study. Firstly we will reclassify the values in t6res_tlc and t6cult. We will use reclassification of Tai Lam Chung Reservior as an example. Switch to the ArcToolBox. From the Spatial Analyst Tools toolbox, Reclass toolset, open the Reclassify tool. Set Input raster as t6res_tlc and Reclass field as TYPE. Change the combination in Reclassification as below. RES Old values 1 New values Name the Output raster to be t6res_tlc_rc (place it in J:\geog4017\lab8). Press [OK] to create the grid. 17. Do the same job for t6cult, set the Reclass field as TYPE. Change the combination in Reclassification as below. CUL Old values 1 New values Name the Output raster to be t6cult_rc (place it in J:\geog4017\lab8). 18. Launch the Raster Calculator (do you still remember how to reach it?). Type this formula in the Raster Calculator "t6risk" * "t6res_tlc_rc" * "t6cult_rc"

10 Set the Output raster to be t6risk_final (place it in J:\geog4017\lab8). Click [OK] to create the grid. Questions: Why this time we multiply the grids instead of adding them together as that in step 16? Suggest another way to perform this calculation (hint: check the steps 17 19). 19. Eventually we have gotten the risk scoring and excluded the reservoir and cultivated land from our study area. The last time we need to do is to reclassify the t6risk_final according to the classification scheme (refers to step 6). (Try to perform the Reclassify by yourself this time. Set the Output raster to be t6risk_class (place it in J:\geog4017\lab8). 20. Right click the t6risk_class layer and select Open Attribute Table. You should be able to complete most of the table by looking into the data in the table. Questions: Complete the table then you finish your works. Landslide Risk Risk Scoring Legend Area (m 2 ) % of total area No Risk 0 White Very Low Green Low Blue Medium Brown Severe Red Excluded area or area without data Black Total Area (m 2 ) 100

11 How can you get the area in square meters for each class? (hint: check your answer of cell size in step 10) How can you get the area of the Class? (hint: check the properties of the grid and you can get the dimension of the raster in Raster Information section) 21. Save the Map Document (File > Save) again. Quit ArcMap (File > Exit). Turn off your workstation before you leave.