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Reproject, then clip or clip, then reproject?

Geographic Information Systems Asked by user112922341 on December 12, 2020

I have a fairly simple question. I have a global categorical land cover map that has no projected coordinate system (i.e. it uses the EPSG:4326 coordinate reference system with no projection). My study region encompasses an international regional geographical scope (for e.g. South Asia). I have to apply a projection to the data because I need the units of measurement to be in metres and not degrees.

My question is this; do I

  1. apply a projection to the global map, then clip it to my region of interest (using a similarly re-projected vector country boundary layer)…or do I

  2. clip it to my region of interest using an un-projected vector country boundary layer (i.e. EPSG:4326) and then reproject the resultant clipped raster to a suitable projection?

I am unsure which method is the proper method, can someone enlighten me? Or maybe they make no difference at all. But just thinking about it, it sounds like the first option is more sound (robust) as opposed to the second, primarily because my input data is a global dataset. Additionally, one reason I am thinking so is because I find it difficult to get appropriate projection systems that cover a regional landscape, but it is relatively easier to find multiple global projections.

One Answer

It depends what is important to you and what CRS the polygon you're clipping it with is in. If you're clipping it using a polygon in a projected system then I would recommend changing them both to the same CRS first.

When we go from a 3D world to a 2D world we have to effectively strech our map. This means that we lose either distance conformity - objects are not the correct distances from a central point, area conformity - polygons are not the correct area or angular conformity - north, east, south and west aren't exactly perpendicular any more.

I would suggest because you're interested in measurement that you want one that maintains distance/area as much as possible.

Answered by DMAD Tim on December 12, 2020

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