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I am planning on conducting a multi-criteria-analysis with a number of different source datasets. A number of the datasets are found in different coordinate systems and projections. Do I need to project these datasets into the same coordinate system before conducting this analysis? My data currently covers more than one UTM Zone, usually they fall within one. Or should all of the data be in the same coordinate system, but unprojected?

Since there is such a large amount of data I would like it to be as efficient as possible.

Although I am utilizing Esri tools for this problem my question is not specific to those. I am working with a dataset which covers 650,000 square kilometres of the NWT and NU in Canada. I've decided to use spatialreference.org/ref/epsg/nad83-nwt-lambert for all of the data. I was wondering if it would be better to leave the data unprojected or pick a projection for all of the data to be in.

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    It might help to name some of the criteria you'll be using as part of your analysis. – Dan C Mar 17 '15 at 20:26
  • Any criteria that depend on metric properties derived from the data--including (but not limited to)--distance, orientation, length, area, angle, and slope--will typically depend on the choice of coordinates and thus require an appropriate projection (or set of projections). For specifics see (inter alia) area, aspect, slope, distance, and general principles – whuber Mar 17 '15 at 20:57
  • @whuber, That's too much work you've put in for a mere comment! – Martin F Mar 25 '15 at 14:24
  • Is this just about how to use esri tools (or which esri tool to use), as suggested by your question tags and by sean's answer? Or is it about the concept of coordinate systems and spatial analysis, as suggested by your wording? If the latter, study the words of wisdom (aka whuber)! – Martin F Mar 25 '15 at 14:30
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    And welcome to GIS SE! – Martin F Mar 25 '15 at 14:31
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The projection chosen for any spatial data analysis is very important.

Your chosen (so far) projection, NWT Lambert, is a conformal projection. Conformal projections are very common in topographic mapping and are ideal when doing land surveying calculations, where measured and calculated surveyed angles are preserved exactly after being projected from the spheroid to the plane. While distances, however, are generally distorted, knowing the properties of the conformal projection, each distance distortion can be calculated and accounted for. Conformal projections will distort areas.

If your spatial analysis is primarily based on calculations and comparisons of areas, you'd be far better off choosing an equal area projection. In that case angles and distances are distorted but sizes (areas) are not.

The ideal spatial reference surface for spatial analysis is the spheroid itself. That is, to use unprojected (spherical) coordinates. However, as far as allowing spatial analysis on the spheroid is concerned, GIS technology is largely still in the Dark Ages: it simply will not do it. So you're stuck with choosing a map projection.

  • This is meant as a supplement to whuber's comment/answer. – Martin F Mar 25 '15 at 15:29
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Not at all. You can set the output coordinate system in the environments setting. Weather that be in model builder, python or even the tool window itself.

I believe it is arcpy.env.outputCoordinateSystems but I'm can't exactly recall.

  • What does "not at all mean"? That the projection does not matter or that it does? And are you claiming that the output coordinate system specification is going to affect the analysis or not? If not, why do you mention it, and if so, why is this true? – whuber Mar 17 '15 at 21:00

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