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While I understand the technical differences between them both, I'd like an expert's breakdown of criteria to look out for that would determine when I should use a PCS or GCS.

I understand that a PCS is an additional algorithm placed on a GCS to transform it from a 3D placement to a 2D placement on a map. For that reason, PCS units are metrics of length (e.g. meters or kilometers) to determine a file's location and shape. GCS on the other hand is its 3D placement on a spheroid, and its units are in degrees, minutes, and seconds.

PolyGeo
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    There's much more than those considerations, a projection is a model and there a few different types each with their own strengths and weaknesses.. however this kind of discourse is off topic for GIS.SE where we deal with specific problems not in generalities. https://xkcd.com/977/ is a humorous but fairly accurate comparison of some of the different types of projections. – Michael Stimson Dec 03 '19 at 03:28
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    How far an area does your data cover? What units are typical for measurements of area / length / position / distance, if any? With that, we only store data for Denver , and some State-wide data, and we store our data in WGS 84 and use PostGIS functions for anything derived in feet / mi / acres / etc. – DPSSpatial_BoycottingGISSE Dec 03 '19 at 03:34
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    It's incorrect to assert that linear (projected) units are "metrics", since linear units also include English units, even if the standard reference unit is meters. It's also incorrect to assert degrees, minutes, seconds, since decimal degrees or radians or gradians could be used (DD reference unit). This topic is quite broad and more than a little opinion-based, and therefore a poor fit for our Focused question/Best answer model. – Vince Dec 03 '19 at 03:42
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    @Vince some projected coordinate systems aren't strong on linear metrics either, Mercator springs to mind, good for maritime charts as bearings are reliable but distances are very rough, the map faces are lozenge shaped so you can't join two adjacent maps together.. this post would probably be more at home at https://earthscience.stackexchange.com or in the chat room. – Michael Stimson Dec 03 '19 at 03:47
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    @Vince - "metrics" means (in this case, at least) "measurable quantities" -- it does not mean SI units. – Martin F Dec 03 '19 at 04:13
  • I vote to re-open because i was in the process of providing an answer. Yes, its a rather general question, so why can i not give a general answer? – Martin F Dec 03 '19 at 04:21
  • The main factors in your decision: the size of your study area, the required accuracy of the data output, and the ability of the software available. larger area and greater output accuracy => GCS. Small area and simple software => PCS. – Martin F Dec 03 '19 at 04:26
  • @MichaelStimson - How is this Q more to do with earth science than with GIS? – Martin F Dec 03 '19 at 04:27

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