New answers tagged area
First, use Define Projection (from the Data Management / Projections and Transformations toolbox). The metadata specifies NAD 83 in decimal degrees, so, GCS_North_American_1983. This gives the shapefile some spatial reference data. After that, you can use Project to turn it into whatever projected coordinate system you want.
The problem is that your data is still retaining the decimal degree numbers, although not units. Subsequent geometry calculations still use the same raw numbers that were associated with decimal degrees, but erroneously assume different units (meters). For example, if I say I'm 65 feet tall, I haven't actually grown, I'm just using different units than ...
You should be able to use the method you were describing above, but create your vector layer using the 'vector grid' tool under the vector>research tools menu. This has a checkbox to align the extents and resolution of the vector grid to a raster layer, so should give you a perfect match with your original raster, and also remember to check the option to ...
1 square metre is 10.7639104 square feet according to Google. Therefore if the cell size is 1 each cell is 1 square metre (1 x 1), the area covered by class is: Count x 10.7639104 if the cell size is 2 metres the area is 4 sq.m. (2 x 2) and the area covered by class is: 4 x Count x 10.7639104 If the cell size is irregular then multiply the width and ...
I believe you would have to create a new column and input the command to calculate what you are wanting. You could try the following: case when "DNAME_2006" IN ('MASINDI') THEN ("Area_1" / "AREA") * "Population" END UPDATE: Unfortunately, I don't think you can do a simple SUM using the Field Calculator. However, there are a couple of other methods you ...
I would check the actual geometry for errors as well. Self intersects can cause differences in reported areas. Run the "repair geometry tool" to find and fix them?
My money is on overlapping polygons. You don't say what software you have access to, but in ArcEditor or ArcInfo (I may never get used to saying ArcGIS for Desktop Advanced, yuck) you could create topology with a "must not overlap" rule and find out that way. (That would also be how to fix it.) Or perhaps easier, fill them with different colors and make ...
Try checking if they are all on the same projection! if they are on different projections their areas may come up as different. You can do this by right clicking your shapefile and going to preferences, then open the tab 'source' and scroll down to Projected Coordinate System.
I think your problem is a special kind of collision detection. You can try to solve it using your own algorithm but it will be very complicated and unreliable specially when your polygons have complex shapes such as wholes. The fastest and easiest way I know for this kind of problems is using the graphic capabilities built into most programming languages ...
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