Calculating Polygon Areas
Problem: You need to get the areas of your polygons
I have often been asked questions like “What is the total acreage of forest in our study area?” or, “Has the total forested area increased or decreased since we last mapped it two years ago?” The answer starts with being able to calculate polygon areas. After that it’s just addition and subtraction.
I used to have to use VBA scripting to calculate polygon areas. Now the task has been simplified with the Calculate Geometry tool introduced in ArcMap 9.2. However, there are some things you need to know to get good results with this tool.
There’s a lot of little parks around the San Francisco bay area in California that will serve as a good test case.
Let’s add the area of each park to the attribute table. The first step is to create a new field to hold the values. Chose field type “Double” (Double-precision floating-point number) which allows lots of decimal places. Then, right click on the field name and chose “Calculate Geometry…” from the context menu. You will then get to choose your units. In this case, decimal degrees are the only option because the layer is in a geographic coordinate system. Decimal degrees are pretty useless.
Fortunately, the tool allows you to use the coordinate system of the data frame, so you don’t have to worry about re-projecting. You just need to change the data frame into a projected coordinate system, one that uses a linear unit of measure, such as feet or meters. UTM (Universal Transverse Mercator) is widely used. This image from wikipedia shows the San Francisco bay area is in UTM zone 10N.
To change the projection of the data frame, select Data Frame Properties from the View menu. On the Coordinate System tab, pick from the Predefined – Projected Coordinate Systems folder.
Now, the Calculate Geometry tool gives you choices like square feet, square miles and acres. I picked acres, a good tiny park sized unit. I got results back with 6 decimal digits, which sure seems good.
But wait. We used a common projection which is great for most purposes, but not particularly for this one. UTM preserves shape and direction, which makes things look right on maps. However, it distorts area and distance. Exactly not what is needed here. The distortions may be small, but if you want to be as accurate as possible (and what GIS Analyst doesn’t?) it is better to use an equal-area projection. Albers projections preserve area while distorting shape and distance. If you root around in the list, you’ll notice there’s several Albers equal-area projections available for different regions, such as Albers North America and Albers USA Contiguous. It is best to pick the most local one you can find. In the State Systems folder, there’s a California (Teale) Albers. Perfect!
Changing the data frame to this, and recalculating the area in a second field, does yield different, better results.
If you need to get the total area of all the parks, don’t dig out your calculator. Just right click the field name to run the Statistics tool.
Tags: area, coordinate system, polygon