Why when is it that when I project a raster from Alaska state plane zone 4 feet to Web Mercator does the file size increase so dramatically? It starts at ~29MB before it is projected and then increases to ~257MB after projection.

NAD_1983_2011_StatePlane_Alaska_4_FIPS_5004_US_Survey_Ft

WGS_1984_Web_Mercator_Auxiliary_Sphere

The only thing I can think of is that the distortion in web mercator at high latitudes is causing the raster to be "stretched" in some way and then leading to an increase in file size.

Thanks for any input.

Forest

I thought I should summarize, although the post by Gabriel Upchurch summarized this pretty well – and your initial assumption [that this is caused by the inherent distortion of Web Mercator at high latitudes] was correct. I should have realized the details sooner.

In the Web Mercator projection, the scale increases as (1/(cosine of Latitude)) so at 60deg it is 2x larger (2x scale error), and your data is at about 70 degrees latitude, so the scale error is almost exactly a factor of 3. Cosine(70) = .3420. That means it is a proper implementation

for a Web Mercator projectionthat a distance of 100 meters will be reported to appear to be 300 meters. I had not realized this distortion would be embedded into a reprojected file, but it is necessary for compatibility with the coordinate system. Multiplied in 2 dimensions, this (3x3) distortio is a factor of ~9, so your input file (29 MB) becomes almost 250 MB after reprojection.This really emphasizes the disadvantage of using a projected coordinate system too far from its origin. If all of your work will be at high latitudes, it would be better to use a local coordinate system, assuming any measurements will be made on the map.

Note I did some testing, and verified that the ArcGIS Online web map compensates for the distortion, so it provides generally accurate measurements. (Exactly how accurate, I do not know – I would need to ask our experts…)

Also note that ArcGIS Desktop has options for how it reports measurements. It will default to "PLANAR" measurements when using the measure tool (which does NOT compensate for the scale distortion), but the tool has the option to choose "GEODESIC" which will provide accurate local measurements.

Options to consider:

or* in raster tile cache format).