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I'm working with 10m DEMs in northern New York, along the Canadian border (in ArcGIS 10.2). To complete a regional elevation map (and eventually slope, flow, etc.), I need to also include a portion of Canada just over the border. While the USGS provides 10m DEMs for most of Canada's border, there is one small section that only has 30m resolution available.

I'm trying to resample the 30m DEM to 10m resolution, but am not sure which order I should work in. The DEMs are in GCS_NorthAmerica_1983, and I'm projecting them to UTM 18N.

Should I project the 30m DEM to UTM 18N, and then resample? Or should I resample, and then project to UTM 18N?

Any suggestions or thoughts on which order would provide more accurate results?

PolyGeo
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KKL234
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1 Answers1

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There should be essentially no difference. I don't know quite how to clearly explain this, but when re-projecting, you are in fact re-sampling at the same time, even if your cellsize were to remain the same. In order to project, the projection algorithm has to know where to 'place' the data, i.e., it has to have a location to project each cell value to, which is probably not the same location as before. This will not necessarily be a simple x,y translation in space, so the way I understand it the data is resampled when projected. If you are using ArcGIS you can accept the default re-sampling method when projecting, or choose a specific resampling algorithm. You may find this post useful: https://gis.stackexchange.com/a/108957/21406.

If you are going to subsequently mosaic all the DEMs into a single raster, then the order is important for other reasons. If you have to extend further into Canada and use DEMs from a non-USGS source, you will also likely have to convert z-values so they are both using either feet or meters. For this case you would (1) Project and resample to common projection and cellsize, (2) mosaic. See here: http://mappingcenter.esri.com/index.cfm?fa=ask.answers&q=1583

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Dan
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  • Thanks for answering. I will in fact mosaic all the DEMs together, so in that case how would the order differ? – KKL234 Jan 26 '15 at 18:30
  • If you read through the second link, they are doing almost the exact same workflow, along the US-Canada border. I will update my answer, but to summarize: (1) Project and resample to same size, (2) mosaic. Hope that helps. – Dan Jan 27 '15 at 00:10