Let's say I have a set of rasters, all generated by the same process, that all have a geographic coordinate system of WGS84, but no projected coordinate system. Therefore, all the cells are in decimal degrees, not meters, etc. Cells have continuous floating-point values (in meters) that I want to avoid resampling if at all possible. The data covers the whole continental United States. I'll call this my "base data".
Here are generally the types of analysis I want to do with my data:
- Take rasters x and y in my base data (both in WGS84, with no projected coordinate system) and add them together. Also, identify the cells in x that have non-zero values which have zero values in y. That sort of thing ... simple map algebra.
- Take another raster from a different data source, which may have a projected coordinate system and/or a different geographic coordinate system, and use my base data to extract the cells in this other raster where my base data cells have non-zero values.
- Take some polygon vector data (again with who knows what kind of geographic and/or projected coordinate system) and compute zonal stats on my base data.
So, my question is, if essentially all I'm looking to do is simple map algebra, where I need to align pixels one-to-one across rasters (or the pixels already naturally align), or simple zonal stats -- basically, analyses where calculation/preservation of shape, area, or distance are not actually relevant -- do I need to worry about supplying projected coordinate systems to all my inputs? Shouldn't I just be able to get everything into WGS84, same as my base data, and carry this out, thereby avoiding having to reproject/resample my base data?
While I'm mainly interested in having the correct theoretical understanding (and keeping my base data in WGS84 as-is), it's also notable that some of ArcGIS's Spatial Analyst raster algebra tools (e.g. "Plus") error out when I'm trying to use them on rasters with only a geographic, and no projected, coordinate system. I can't tell if this is a software quirk of Arc, or if it's trying to tell me that what I'm doing is inadvisable conceptually.