Category Archives: FORT 260

Extensions need to be authorized

Recently, I wanted to use the Spatial Analyst extension on ArcGIS 10 and got this cryptic message.


I knew for certain that the extension was licensed. It showed as installed and licensed elsewhere in the program. After getting help from the campus IT techs we found that the extension had to be turned on by going to Customize –> Extensions and filling in the screen shown below. Then everything will work properly.


This is similar to the situation in Excel where the statistical toolpack has to be enabled in the program properties screen separately in order to work. I just wish ArcGIS had made a more explanative error message! 

How much area do buffers on streams take

In this exercise we clipped streams to the borders of the Michaux Forest. Then we created buffers of different widths to see how much area each buffer width takes up.

StreamBuffersHarvesting.jpgBelow are summaries of the lengths of the streams by use categories and the total area taken up by each of the buffer widths.

Stream Lengths by Use Type Feet
Square Feet               Acres Square Miles
25 Foot Buffer 47,954,927 1100.89 1.72
50 Foot Buffer 96,952,489 2225.72 3.48
100 Foot Buffer 197,985,925 4545.13 7.10

The personalities of map projections?

map_projections.png I never knew that map projections had personalities…

Image permalink.

Stream Buffers

Protection of streams is critical to maintaining water quality. In Pennsylvania quality standards for streams have been set in Chapter 93 of the Pennsylvania Code. Below is a map of the Michaux State Forest showing streams going through the forest.


Clipping the streams to the forest boundary shows how much of the forest covered by waterways. The following map shows 200 foot buffers. MichauxStreams200ftBuffers.jpg A detailed view shows the relative difference in size of 100 foot, 150 foot, and 200 foot buffers.


At first I thought that increasing the buffers would have a large increase in the area covered, As the following table shows the increase in area is linear with the increase of the size of the buffer. The table also shows the total area of the forest and the total length of the streams.

summary Table.jpg
This data was analyzed with ArcGIS 9.3. The data comes from PASDA.

Develop an Elevation Profile

In surveying we have been making elevation profiles from differential leveling data for as long as I have been teaching the course. Recently, I came across a help page from the University of Georgia that uses ArcGIS to associate data points with elevation data from a DEM, which is then graphed in Excel, with an XY scatter chart. The data can come from GIS points or a line digitized on the map. The edit – divide tool sets the points along a line at any spacing desired. Spatial analyst will extract the elevation from the DEM at each point.

Here is a section of the pipeline that goes through the Waynesboro Watershed.
And here is the resulting profile.
This method can help reduce the error form field measurements. And it can be used for much longer spans. 

Use GIS to study cover type changes

The Penn Pilot web site provides historical aerial photos for most of Pennsylvania. You can either view the photos on the site or download them to use with other applications. Recently, in GIS class we used ArcGIS 9.2 to position the old aerial photos from 1937 over a current map of the southern portion of the Waynesboro Watershed using a process called georeferencing. Much of the area that is now the reservoir and its watershed was actively farmed. Here is an image of the map layout showing the farm fields outlined in green. (Click on the image to see a full screen version.)


When we remove the overlaid aerial photo we can see what the land looks like today with just the bouhdaries of the old fields. It’s evident that when forestry students in the 1950’s started working in the watershed they put the pine plantations on the old fields. This makes a lot of sense considering that it is much harder to put a plantation in existing forest.
historic_watershed2.jpgFor instructions of how to perform this operation click here. To use Penn Pilot click here.

Lessons Learned

The more I use ArcGIS the more I realize that it is a very complicated piece of software. You have to be careful to understand each function to deal with problems as they arise.

Here are a few tips I have learned the hard way:

  • Always make sure that the projection of the data frame is defined. This sets the stage for all new data that is added.
  • When exporting data from one data layer to a new data layer choose to put the new data in the data frame projection. This really helps when using data from the PASDA GIS server. It will also help when doing calculations.
  • Always try to put data in a geodatabase rather than leave it in a shape file. The geodatabases recalculate the shape area values by default.
  • When you want to determine the areas of polygons in a data table there is a calculate geometry option that lets you put the areas in any units, not just the data frame units.
  • Take advantage of the export map option to use a map document as an image file, like this map below.

  • Project_Boundary.jpg

GIS and Resouce Conflicts

During yesterday’s discussion on the uses of GIS the author, Bolstad, emphasized that GIS can be used when there are conflicts in the use of natural resources. Or where there is competition for resources. This got me thinking about the tragedy of the commons. This was the title of an article written by Garrett Hardin back in 1968. He describes a scenario where a natural resouce, such as a common field used for grazing will almost always be degraded since no single user has the authority or incentive to ration use of the common field. I’m sure you can think of other situations that result in the same unhappy situation.

One solution to the commons problem that has been suggested is turning the common resource into privately owned resources, such as partitioning the field for individual owners. Another is to create strictly enforced rules that ration the use of the commons so that they can be used sustainably by all. Both of these solutions have their own advantages and problems.

GIS can be used to analyze the resource, determine where the conflicts occur, illustrate possible solutions, and document results over time. GIS let’s us run what-if scenarios and document how things turn out.