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Here are the layers that I have:

  • State / Province polygon file (With Area Field)
  • County / Ward polygon (with no area field), but all the polygons are within the state boundaries
  • Water ways for the whole province / state. (includes rivers, lakes, creeks, brooks, etc) with no area fields

Here is what I want to do:

  • Determine the gross land area of counties without the rivers and lakes.

Notes

  • The area doesn't have to be extremely precise (nearest 5 sq km is fine with me)
  • Has to work on ArcMap or QGIS
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3 Answers

clip analysis in Arc Tool box of arcgis all versions

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

In an arcmap solution, I would first move all features to a geodabase. This would automatically add geographic fields such as length, area of features.

I assume that the waterways are a polygon feature and not line features since there are lakes in it.

I would union the county and water feature to slice up the former layer with the features in the latter and adds attributes of both. Benefit is you have a full layer, including waterways, but they are segmented into water and not water features for later analysis.

Then you can select water features in the new county layer and either delete them or definition query around them.

You can then do your summary of the county area, sans waterways.

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If your hydro features are polygons you could use the Erase tool to erase those areas from the county features. Then you could just update the county features using the calculate geometry option within the attribute table. I think this is what you mean by, "Determine the gross land area of counties without the rivers and lakes"?

Erase Tool

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Especially since you are dealing with large areas, make sure that your output is in an equal area projection. The erase tool will reproject your inputs on the fly, but you might even want to do a preprocess reprojection so you have more control over which transformations are used, if any. –  blord-castillo Jul 29 '11 at 17:22
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