The topic of re-projections vs coordinate system transformations keeps coming up lately so it was suggested that a post be made. Since many of us use our local county data, maintained in the same spatial reference system, re-projecting and transformations are rarely needed or thought of……with some exceptions.
ArcMap will re-project on-the-fly (e.g. from Conformal Conic to Mercator and vice-versa) but it won’t perform a proper coordinate system transformation automagically (e.g. from NAD83 to WGS84 and vice-versa). If you start with one layer, say parcels in the standard NAD83 State Plane Z-V coordinate system and Conformal Conic projection, and add another layer, say the LARIAC5 WMTS imagery service in its WGS84 coordinate system and Web Mercator projection, you should see the warning message here (unless you disabled it).
If you ignore this message (by clicking “close” or the “X”) then your layers may display a shift of 4 to 6 feet on average in Southern California. I say “may” because the original NAD83 and WGS84 datum are said to be positionaly identical. I say “shift” because “align” is relative. If you don’t see the message, then you would still need to apply a transformation or just know that your data is misaligned (I’ll leave it to you to know if and when a few feet matters). You can quickly test this by opening two instances of ArcMap, adding the same two layers mentioned, transforming in one instance but not the other, and then observe the shift and even measure the difference. My version of ArcPro doesn’t seem to give the warning, but my understanding is the behavior is the same as ArcMap and you would need to perform a transformation when it matters.
Simple right?
The topic has been coming up as developers provide new subdivision data in various formats that may not contain spatial reference information (or metadata that ArcMap recognizes anyway). Dragging a dwg file into ArcMap, without related spatial reference, gives an even more cryptic message (ArcMap draws it but doesn’t project it). This leaves us to make assumptions and guesses about the data.
We may assume that this data is in our local coordinate system (NAD83 CA Zone V of course) and then specify that assumption by defining it for ArcMap. However, that may not be the case as the data may represent another coordinate system or represent ground measurements that require additional ground to grid conversions (I’ll defer to the civils and survey types on this one). In addition, NAD83 has several generations of realizations/adjustments/epochs in both feet and meters. Mixing the original NAD83(1986) coordinate system with a more current NAD83(2011) 2010.00 realization should, you guessed it, give you the “Geographic Coordinate System Warning” that started this whole discussion (the difference here should be much less severe than the NAD83 to WGS84 example).
Bottom line, make sure you’re working with the same apples or are at least aware of the difference in those apples when it matters.