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I have an sdc file with water bodies, i.e. oceans-rivers-lakes. I want to combine it with a shapefile with the boundaries of countries all over the world and produce two measures: 1) The average distance from waterways for each coutnry, and 2) the average fraction of land that is within e.g. 100km from water.

I work with ARC GIS 10.1.

Any suggestions on how to proceed?

Thanks!

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1 Answer

I only have access to ArcGIS 9.3, but I expect the procedures won't differ that much.

Your first question is better solved using a raster (if you have spatial analyst) approach:

  • Using Spatial Analyst Tools > Distance > Euclidean Distance, create a raster with the shortest distance from any river\lake in the "world" (don't forget to set convenient cell size ;
  • Using Spatial Analyst Tools > Zonal > Zonal Statistic, calculate the mean distance for each polygon in your "world boundaries" shapefile;

Your second question can be solved with a vector approach:

  • If your country shapefile does not have it already, add a field and calculate each country area by using "Calculate Geometry..." in its attribute table;
  • Using Analyst Tools > Proximity > Buffer, create a 100 Km buffer around your river\lakes shapefile (you should use the dissolve option ALL);
  • Use Analyst Tools > Overlay > Intersect, to combine both shapefiles;
  • On the resulting shapefile, add new field for new_area, and calculate it using the process described before;
  • Add one last field to your table called percent, and using Field calculator, calculate 100 * [new_area] / [country_area];

Hope it help.

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THanks! It sound very feasible and I ll try it instantaneously. One more question. As to calculating the area do I need to somehow to weight the cells for each given latitude? i.e. the cells near the equator represent a larger are than the cells near the pole. –  An_Li Mar 18 '13 at 13:38
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To calculate areas, for you should consider using a equal-area projection,say Lambert Cylindrical equal area. That way, area calculations will not suffer from distortions. –  Alexandre Neto Mar 18 '13 at 14:29
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uhmmm.... Now that I think about it, world measurements need a bit more care. The distance to river\lakes measurements also need to be used with some extra attention, as projections might add distortion the real distance between points. I'm not sure if ArcGIS handle the distance measurements correctly. –  Alexandre Neto Mar 18 '13 at 14:38
    
+1 Good answer and good thoughtful follow-up comments, too. One method to cope with distance distortions in worldwide grids is to compute the distance grid as accurately as possible (perhaps by doing it in overlapping local regions using their own good projections, then reprojecting and mosaicing the results), then reproject that grid to an equal-area grid for the purpose of computing averages. –  whuber Mar 18 '13 at 15:20
    
Thanks for all the comments. I ll will try both and see if they generate significan differences. –  An_Li Mar 18 '13 at 15:57
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