# Creating Latitude Grid from DEM

I am trying to evaluate an equation in the raster calculator and I need latitude values. I have the DEM for the region and am wondering if it is possible to create a latitude raster based on the DEM?

• What software... ArcGIS? Is your DEM projected? Aug 11, 2011 at 12:14

Unproject the DEM to determine the geographic region it covers.

Create the latitude coordinate grid for this region. (In Arc* products prior to version 10, this is done with "\$\$YMap": http://forums.esri.com/Thread.asp?c=93&f=1740&t=182407.)

Projecting this grid produces the latitude raster. For the projection, use some form of interpolation for the resampling (bilinear should be good enough in most cases), not nearest-neighbor.

whuber's method is simpler, but this should also work if you have Spatial Analyst and ArcGIS:

1. Spatial Analyst Tools > Extractions > Sample, to create a points layer for each cell
2. Add X and Y fields to the points layers and calculate the X and Y values using "calculate geometry"
3. Use Conversion Tools > To Raster > Points to Raster to create a new grid for each value.

I realise this is an old thread but thought I'd contribute to help anyone searching in the future. My DEM was too large to use Patrick's method, so I used a points file that contained a reasonable number of points (approx. 70, spread across the extent of the United Kingdom), then interpolated between these (using latitude as the z value). Hope this is helpful to someone, sometime!

The simplest way to create Lat/Long grids rasters is to use the `r.latlong` function in GRASS GIS.

By importing a DEM (or another raster), you can use the DEM as input and the function will create a latitude (and longitude using the `-l` flag) that matches the input DEM. You can then export to any desired format. This took ~ 1.5 min for a 13 x 10 degree, 30m DEM, using a 32 GB RAM, 32 Core workstation.

https://grass.osgeo.org/grass74/manuals/r.latlong.html

I used Kate's approach in ArcGIS (very much appreciated, even after all this time). I used a basic IDW interpolation with a variable search window on ~300 points. The rabbit hole that led me this far has been trying to ADD a directional elevation trend to to an existing DEM dataset to impose a stronger flow accumulation trend in a flat landscape. As the \$\$YMAP/\$\$XMAP GRID variables are no longer supported in Arc 10+, this was the easiest workaround for me. Doing this is was able to create two rasters for X and Y coordinate values to then work into map algebra.

This is now trivial to do with GRASS tools available within QGIS. Open your raster data in QGIS (e.g., 'raster_dem.tif') and open the GRASS tool `r.rastercalculator` in the Processing Toolbox. For the `formula` variable, enter this:

``````raster_dem = y()
``````

This will produce a raster of latitude values. If you also need a raster of longitude values, simply use `x()` instead of `y()`. It's much faster than previous methods too.

• It can be kind of dicey to do this through QGIS, because the region creation isn't reliable; this method didn't work for me. It did work to do the whole thing in GRASS, but I had to first double check & reset the region with g.region (or Settings -> Region -> Set Region in the GUI).
– Nat
Nov 18, 2016 at 22:26