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I am looking to calculate the percentage cover of different land classes in a number of districts in southern Malawi. The land cover map (raster) can be found here - http://www.masdap.mw/layers/geonode%3Amalawi_landcover_2010_schema_2, and the boundary data can be found here - http://geoportal.rcmrd.org/layers/servir%3Amalawi_adm2

I have used "tabulate area" in spatial analyst tools which seems to be what I need - I now have a table with values for each land cover type in every district. However, the values are very small (most are 0.0001 for example)... can anyone tell me what units this is in? My map is 30x30m resolution.

Also, once I have figured out what units the area of each land cover is, can anyone tell me how to find out the total area of each district? This will allow me to figure out % cover.

  • Do you know which projection your data is in? e.g. you could check the properties -> source tab or when you place your mouse cursor on the map canvas what coordinate values units do you see in the bottom right of your map. – whyzar Oct 10 '17 at 13:46
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Based on the information you provided, resolution, my guess for your units is meters. Also, to find the total area of each district you could follow this tutorial Calculating Polygon Area in ArcMap

You will have to create a new field then calculate the area of polygons, it is straightforward workflow

Select the “Open Attribute Table” to open up the associate attribute data for your polygon layer.

In the upper left hand corner of the attribute table window, click on the Table Options icon to open up the menu options. Select the “add field” option.

To be able to calculate area, you will need to work with projected GIS data. There are two ways to do this: either load in projected data or load the data into a data frame that has a projected coordinate system (PCS) assigned to it. To set a PCS on a data frame, right click on the layer icon and select “properties” from the drop down menu. Then select the “coordinate system” tab and select a coordinate system from the options.

Select the unit of area of interest

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Tabulate area is the right tool for this job, but it yields the area expressed in the coordinate system of your data. In your case, the coordinate system seems to be a geographic coordinate system (Latitude/longitude, e.g. WGS84). Indeed, your small values indicate that your unit is likely to be degrees.

Therefore I suggest that you "project" your data in order to have a metric area measurement. UTM36S could be a good choice for MALAWI, but Africa Albers Equal Area Conic is even better because you are doing area computation. Finally, make sure that your analysis cell size (in the environment settings) is set to 30 m because ArcGIS may select a coarser resolution by default.

  • Thanks for your reply! I have changed my coordinate system to Africa Albers Equal Area Conic - but when I changed my cell size to "30" this didn't work. Perhaps I have not followed the right steps? I used Geoprocessing > environments > raster analysis > cell size "as specified below" and then entered 30. Is this correct? – C.Hall Oct 10 '17 at 15:12
  • @C.Hall Which tool did you use to change the coordinate system of your data? Project or Define Projection? – Dan C Oct 10 '17 at 16:01
  • I used project. I have now got it working - I think! The numbers are much more reasonable, for example, dense forest cover in district 1 is 13934.221041. I am assuming this is now in metres squared? – C.Hall Oct 10 '17 at 16:58

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