I'm trying to get the percentage of each U.S. county overlaying groundwater (data from the census and USGS respectively). They're both polygons.

I have 1) correctly projected the data and the layer (NAD 83, meters) 2) clipped the counties using a polygon corresponding to groundwater coverage 3) use calculate geography to obtain the areas, so that I can then figure out proportions/percentages. I create a new field for that (double).

I've been toying around with this and other methods, and I cannot get credible area values. They're tiny; almost all 0.01 - 0.1. One of my many hunches was that it's in decimal degrees, but I've projected the data, and it's still showing up incorrectly. The most frustrating part is that the county/state files won't calculate correctly either on their own, which is bizarre (Florida = 15 m2).

Any ideas?

  • 1
    If you measure the side of a polygon (e.g., Florida) with the Measure tool, what's the size in meters? If you overlay the polygons on one of the basemaps from Esri, do they look like they're correctly sized/located? Also, what are the area calculations for the state or county polygons before projection?
    – Erica
    Jul 25 '14 at 11:47
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    You describe the data as being in the "correct" projection, but you don't say what that projection is (NAD83 is a spheroid and a GCS, but not a PCS).
    – Vince
    Jul 25 '14 at 11:58
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    Would you be able to edit your question to include the GIS software and version that you are using, please? It sounds like you may be defining lat/long values as PCS instead of projecting lat/long values from GCS to PCS. More details about your precise steps will help.
    – PolyGeo
    Jul 25 '14 at 12:10
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    What is the range of values in the map extent? It seems likely you defined the projection instead of actually projecting it, so the data remains in degrees.
    – Vince
    Jul 25 '14 at 12:34
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    Are you using the Project tool in the Toolbox, or Define Projection? (Project is what you want, it actually transforms data into a new shapefile.)
    – Erica
    Jul 25 '14 at 12:35

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 inches -- and the measurement is wrong even though the number is right.

The Define Projection tool only changes the associated projection, it does not modify the geometry of the feature or shapefile. (Useful if you get data with no associated projection. Not useful if you're trying to transform to a different coordinate system.)

Use the Project tool in the Projections and Transforms / Feature Toolbox, which will mathematically transform the data into a different coordinate system.

If the data projection is changed without transforming the data, it will look straight along the top (as Vince pointed out in the comments above) and a little warped. If it is transformed, it will look curved.

Side-by-side comparison of original and transformed census data

You can also check whether a projection is correct by creating a new data frame, adding in a basemap, and then adding in the data. In this case, I used Define Projection to say the coordinate system should be NAD83 Albers (Contiguous USA), and then put it on the Oceans basemap. The entire US is in the Gulf of Mexico, and is the completely wrong size -- it's that tiny dot 100 miles east of Ciudad Madero.

Tiny dot in Gulf of Mexico where incorrectly projected data went.

  • 2
    +1 for beating me to the punch, but adding two quick graphics with US counties unprojected and projected would help.
    – Vince
    Jul 25 '14 at 12:51
  • Good point @Vince. I'll add that.
    – Erica
    Jul 25 '14 at 12:55

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