I'm using land use data, and report the proportion of each sub-catchment that is used for “Grazing Modified Pastures” and “Irrigated Modified Pastures”.

screenshot of layers

I'm essentially focused on these two layers, I need to know how much of catchment area falls within Grazing Modified Pastures and Irrigated Modified Pastures.

What is the best way to process this in ArcMap 10?

  • 2
    Do these two layers overlap, or not? – Erica Aug 24 '14 at 3:23
  • 3
    You can do this with Union (Analysis) and then summarize by catchment. – Aaron Aug 24 '14 at 3:26
  • They don't overlap, per se, each has their own attributes and areas, i.e. landuse just simply has areas with uses of land (yellow) and Sub-catchments just has catchment type, and area. – Jan Aug 24 '14 at 3:28
  • As mentioned by Chris W, use Intersect. After this the most efficient approach is using Pivot Table in Excel, where catchment IDs become row, field storing Irrigated and Modified shoul be column headers and areas are the values to be summarized – FelixIP Aug 24 '14 at 23:08

Intersect, or Union as Aaron mentions, are Overlay tools that should get you a set of polygons that have attributes of both. Identity is also an option if you have an Advanced license, and is probably the most suitable for your end goal. Basically you want to cut up land use polygons if they cross a catchment boundary and then assign the catchment id to the resulting shapes while preserving the grazing/irrigated attributes.

Note that Intersect only returns areas of overlap, Union returns all areas from both layers, and Identity will return all areas from one layer but cut up according to the other. The attribute format/output from each tool varies and one may better suited to your problem.

Once you have have the new polygons, you can run Summary Statistics on the attribute table using catchment ID and whatever the grazing/irrigated attribute fields are called as CASE fields (note field order matters to resulting table), with your area field as the STATISTIC field using SUM type. This gives you a table with a breakdown of catchment, pasture type, the number of polygons that comprise the unique combinations of those two attributes (frequency), and the sum of the combination's total area.

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