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I am doing a series of geoprocessing steps in ArcGIS with a couple different datasets, for a global analysis of habitat loss and impacts on biodiversity. I have raster forest cover tiles and polygon range data (unprojected and/or in the GCS). I have my series of geoprocessing steps figured out but I am wondering if it is better to project the data early on in the process or later on. For the rasters I figured out that I couldn't project the tiles prior to mosaicing them - otherwise there were gaps in the mosaiced output.

Is there a standard or best practice or does it even matter?

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    For all the people voting to close as opinion based - it might sound opinion based, but the answer given by @Fezter is proven with facts (re: projection and accurate results). IMO nothing wrong with these best practice (seemingly opinion, question). Why not consider if a thoughtful answer can be given before jumping to close a question. – KHibma Apr 17 at 12:22
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When you are doing any type of analysis, it's best to use projected data, in my opinion. For example, if you're analysing distances, it's best to use a projection that preserves distance so that you get accurate results. If your data is in a geographic coordinate system (Latitude, Longitude), when you measure or calculate distances, the results will be in degrees which is non-sensical as the distances between longitude varies with latitude.

Similarly, when you are doing analysis that requires correct areas, it may be best to use an equal-area projection.

In my experience, when using rasters for analysis, it's important to use a projected coordinate system. For example when performing a slope analysis from a DEM, you'll get incorrect results if your data is in a geographic coordinate system.

As far as when to perform your projection, I suppose it would depend on what particular geoprocessing steps you're undertaking. If any of them rely on the units of the data, then I would project before that step. If none of the steps rely on the units, then it may not require projection at all.

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As you have found out, there is at least one scenario where the sequence order in which the data is projected matters, and that is when you mosaic (aka merge) raster tiles. In that situation you must mosaic before projecting. Otherwise, gaps appear between the mosaicked tiles.

I'm not aware of any other sequence-dependent projection issues, but my experience may be limited; others may have additional information.

However, I'm particularly interested in the second question of your post's title, "Or does it even matter?" I suppose that depends on what you mean by "it", so I'll rephrase the question as "Or does establishing a project Coordinate Reference System (CRS), with associated projections and transformations, matter?"

To that question: If your project requires any analysis (buffers, intersects, spatial joins...) then you must establish a single, appropriate, project-based CRS up-front, and project any data that will be subjected to analysis to that CRS a priori.

Why? Because analysis tools rely on the internal location values stored in each layer when doing their computations. The tools will happily do their job, even with layers containing wildly incompatible values (say, merging one layer with UTM meters to another layer with Lambert Conformal feet). In such cases the output is likely to be non-sensical.

GIS staff are often confused when this analysis problem occurs because modern GIS software will display layers with divergent CRSs so that they line up, using "on-the-fly" (OTF) projection, and users thereby assume that the results of data analysis will also magically line up. Not so! Early GIS software did not have OTF display projection capability. Thus, when two layers with different CRSs were displayed on-screen, you could clearly see that they did not line up, and intuitively knew that any analysis with those layers would result in problematic output.

The arrival of powerful desktop computers allowed for OTF display of data with differing CRSs. Although useful for display and map production, OTF no longer provides the user with visual clues about the underlying data, lulling them into thinking that analysis will likewise line up, resulting in a huge source of GIS/mapping confusion.

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  • Thanks. I think my question of does it even matter was a bit unclear. I meant the stage at which projection occurs doesnt matter. Obviously projection matters. – user161940 Apr 17 at 18:49

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