Warnell GIS and Spatial Analysis

Earlier this week, a student asked me how to create an elevation profile for the trails on their (Senior Project) client's property.  An elevation profile is a plot showing elevation across space (along a trail, road, rout, etc.) and should be fairly easy to create in ArcGIS.  All you need is a DEM, a line shapefile (your GPS'd trail), and a point shapefile. 

You can grab the DEM from the GIFF spatial database.  I'll assume you already have the line shapefile either from digitizing the route from an aerial photo or from GPS.  You will have to open ArcCatalog and create a new point shapefile.  Don't forget to assign an appropriate coordinate system.

(I'm using the Rabun county DEM, 'rabun_dem' and the Rabum Bald NE 1999 DOQQ 'rabun_bald_ne.sid' from the GIFF database - the p:\ drive in WSFNR labs)

My line shapefile is called 'LineFeature' and the point shapefile is 'PointFeature'.  A screenshot of my trail is below. 

The following will step you through creating points at an interval along the trail, assigning an elevation to each of those points, and graphing them out to produce the profile.

1. Start Editing 
2. Select one feature in LineFeature
3. Set PointFeature as your Target in the Editor menu
4. Click Editor > Divide 
5. Tick the button next to 'Place points separated by every__ units' (coordinate system units; UTM=meters, State Plane=feet)
   
6. Hit OK
7. Editor > Stop Editing; Save Edits YES

Look at the attribute table of PointFeature.  The First point is at the beginning of the line.  The 2nd is 100 meters from the starting of the trail.  The 10th point is 1000 meters from the beginning; etc...  Create a new field to store this information.

1. In the attribute table, Options > Add Field...
2. Name: = TrlDist; Type: = float; OK
3. Right-click on the new field > Field Calculator...
4. [FID] * 100
5. OK

Now you need to add the elevation values from the DEM to the attribute table.

1. Open the 'Extract Values to Points' tool (ArcToolbox > Spatial Analyst Tools > Extraction > ...
   
2. Input point features = PointFeature; Input raster = DEM; Output point features = specify a new shapefile
   
3. Check the box next to 'Append all the input raster...'
   
4. Hit OK
   

Open the attribute table of the new shapefile.  Notice the new field 'RASTERVALU' - that is the DEM elevation at that point.  If some points don't have a value, you might have to check the 'Interpolate...' box in the Extract dialog.

Export the attribute table as a new DBF (Options > Export from the attribute table), open it up in EXCEL, and create a scatter plot.  The X-axis is the trail distance and the Y-axis values are the RASTERVALUs.

 While you are in excel, you can calculate the min, max, and slope (rise / run).  You should include that information in addition to

• the starting and ending X and Y coordinates and
• mark or label unique features, and
• mark or label the problem areas (large slope, wet, etc).