Exercise 2: Working with Vector Data in ArcGIS 9.3 There are several tools in ArcGIS 9.3 used for GIS operations on vector data. In this exercise we will use: Analysis Tools in ArcToolbox Overlay Analysis (Clip, Intersect, Union) Proximity analysis (Buffer) Calculate Geometry Update area and perimeter, Add X-Y coordinates to a table Analysis Tools in ArcToolbox Step1: Start ArcMap with a new project. Add the following data layers located in c:\nr502\idaho_utm Data Layer Description Source Scale Boundary Idaho boundary Inside Idaho County Counties of Idaho Inside Idaho Road_100 Highways, roads and trails USGS 1:100,000 Stream_100 Streams USGS 1:100,000 idstwrd_utm Idaho ownership GAP data 1:100,000 We will use this base data to create a GIS dataset for the Craig Mountain Wildlife Management Area located south of Lewiston, Idaho north of the confluence of the Snake and Salmon River. Step 2: Add the Craig Mountain boundary to ArcMap. The boundary is a shape file located in the folder c:\nr502\craig and named craig_bnd.shp Step 3: ArcToolbox and open the Analysis Tools. 20
Clip CLIP operation: Clip one layer based on another. The CLIP operation works like a cookie-cutter. In this exercise we will clip streams, roads and ownership to the boundary of Craig Mountain. Step 4: Select Overlay Extract-Clip in the Analysis Tools menu. Select road_100 for the input layer. Select craig_bnd for the polygon layer (cookie-cutter) Browse to an output location and name the output data layer. Step 5: Use the Clip function to clip out streams (stream_100) and then ownership (idstwrd_utm) using the Craig Mountain boundary as the clip-layer. After you have created the subset layers of roads, streams and ownership you can remove the statewide layers from the project by right-clicking on the layer and select remove. This removes the layer from the map project, however the data file still resides in c:\nr502\idaho_utm on your computer. Update Area Remember using the to Calculate save your Geometry work periodically! function After overlay (intersect, union) or extraction (clip) operations you should always update the polygon area in the shapefile. New polygons are created through these operations and the area is not automatically updated. a) Open the attribute table of the shape file you need to update area for. b) Right-click on the area attribute in the table and select Calculate geometry c) In the next dialog box you can select the coordinate system and the units for the area calculation. To be consistent with the data you are working on, choose the current coordinate system of the layer and the units that correspond to this coordinate system. For example if you are working in UTM coordinates, select meters for the units (UTM is a metric coordinate system). 21
Create a summary table in ArcMap Step 6: Open the theme table for the ownership layer for the Craig Mountain ownership layer. Make sure the area has been updated according to above instructions. The attribute OWNERNAME designates the land-managing entity (BLM, Forest Service, Private etc.) for each one of the over 200 polygons in the table. In the next step you will create a summary table that summarizes the area for each owner category. Step 7: Right-click on the attribute field OWNERNAME. This is the field you want to summarize data for. Place a check mark in front of Sum in under Hectares. This means that you will summarize the number of hectares by ownership category. Name the output summary table. This table is part of the deliverables for Lesson 2. Save the table!!! Intersect The INTERSECT operation is an overlay analysis between two vector data layers that creates an output data layer that contains the spatial and attribute information from both the input layers. For example, if we intersect the Craig Mountain ownership layer with the Idaho counties we create a layer that contains both the information about ownership and what county each polygon belongs to. 22
Step 8: Use the ArcToolbox Analysis Tools to intersect the Craig Mountain ownership layer with the Idaho counties. Select Analysis Tools Overlay Intersect in ArcToolbox. Step 9: Craig mountain ownership and the Idaho counties are input features to the intersect procedure. Name the output layer. Step 10: Notice how the arcs from the county layer have been incorporated into the ownership layer. The polygons along the county boundary have been split in two one that resides in Lewis County and one that resides in Nez Perce County. The attribute table for the resulting theme contains information about both ownership and what county each polygon resides in. Step 11: Update the area for the intersected data layer using Calculate geometry. Why is this necessary? Export to Excel ArcGIS tables can be exported to Excel by selecting Export in the Options menu on the lower bar of the table. The tables are exported in.dbf format that can easily be imported to Excel, Access or other database software. 23
Proximity Analysis In natural resource applications proximity analysis (buffering) is often used to estimate area of riparian zone and habitat along streams. Roads, shorelines and legal boundaries are other features commonly buffered. Point locations can also be buffered. In analyses of wildlife habitat it is often of interest to determine what habitats are within a certain distance of animal observation locations such as nests, foraging areas, etc. In the following exercise you will buffer the streams on Craig Mountain and calculate how much area is within 30 m of a stream. Step 12: Select Analysis Tools Proximity Buffer in ArcToolbox. Buffer the streams on Craig Mountain with a 30 m buffer (make sure to buffer the streams on Craig Mountain rather than all of the streams in Idaho!!). Step 13: Use Xtools to update the area for the stream buffer. How much area on Craig Mountain is within 30 m of a stream? Save the project. You will be using this project in future lessons! Deliverables Lesson 2 1. A map displaying the land owner categories on Craig mountain. Also include roads and streams on the map. Include a legible legend, scalebar, northarrow etc. 2. The summary table of area in different land ownership created as part of this lab (in Step 7). 3. An estimate for how much area is within 30 meters of a stream on Craig Mountain. 24