I made a couple hundred polygons in Google Earth and brought them into ArcMap, but I could really use some help understanding all the attribute table entries and how to change things so the "area" is actually useful.
This is what I have imported. Can someone tell me what the different attribute table entries are? Why do I have 6 different symbol IDs did I by chance do different polygon outline colors/sizes? Is shape-length the length of the polygon border?
Then comes the most important, area, why is one <null> and the other so tiny. Is the shape area in some inappropriate units? How can I fix that?
Appreciate any help.
The coordinates of the data are in decimal degrees, hence the areas are in units of the useless square decimal degrees.
If you want planar area, then use the Add Geometry Attributes tool from Arctoolbox
specify a projected coordinate system for your area (eg UTM etc) and
specify the area units using something useful like km sq
That certainly sounds right, but it didn't add anything to my table despite saying it completed. Did I not enter something correctly? The DEM I'm using is in WGS84 UTM Zone11 so I used the same projection, and it didn't give me an error.
Any idea what the other entries are in the table? I want to know if I can safely delete them or if they are useful, and why the symbol ID changes.
Make a projected version of your polygon file to get one in the actual coordinate system.
Your units will be in meters (and meters squared for area), Use add geometry attributes if you want km^2 or do the math
Oh that is excellent. Is shape length the length of the border of each polygon? Any idea what those middle table entries are? I feel they must be Google Earth elements, but they clutter up my table and if I can lose things like "snippet" and "popupinfo" and the Null columns it would be neater.
Also this is beyond the scope of the original question, but if I make a slop map, then use Zonal Statistics to assign each polygon an average slop value, can make that a column in this table? Then can I use that slop to correct the map view area to a surface area?
Shape length is the "perimeter" of the polygon border.
You can delete unnecessary fields or simply hide them in the "fields view" of the table.
As for getting the 3D area, unless the slopes in your polygons are very large and spatially irregular (think mountains), you won't find a large difference. In any event 3D tools are better suited to this task, for example
@Atlas_Beetle the fields coming out of Google earth are all useless apart from the one containing the useful name of each feature created. You can safely delete without discrimination 👍. The keyhole markup language handles attributes differently to shapefiles and feature classes.
As @DanPatterson says, project your FC, then just add geometry attributes.
If you use the Zonal Statistics as Table geoprocess, you can then join the table to your original layer and then either leave like that or field calculate the slope values across to a new field in your main dataset 👍
How would I go about making a simple correction from map area to surface area. Is there a way to generate a new table that calculates the result of my map-area values divided by their respective cos(average slope)?