I’m designing sundials and building models of the designs in Sketchup. A given design will work fine in some locations (i.e., Midwest US) but will be off by several minutes in others (i.e., Western US). As a check, I imported a location into the free web version of Sketchup in the western US to set the geographic location. I chose a location directly on the 120 W. meridian, the baseline for Pacific time. I erected a vertical pole, turned on shadows, and set the time to 12:00 and the date to September 1; this should result in the sun casting shadows directly north, but instead it casts a shadow slightly to the east; backing off the time to 11:55 am renders the shadow correctly. September 1 was chosen because the Equation of Time on that date is very near zero.
I noted, too, that the imported map imagery is rotated a few degrees clockwise - it looks like this is the same angular error I’m seeing in shadow direction.
There are similar but smaller errors at other locations; in Michigan, the error is about one minute.
All documentation states that the green axis in Sketchup is aligned with true north, but this is not the case. I suspect Sketchup is using a poorly georegistered map for its web version, but I’m not certain.
Note: I can easily rotate the model itself if that’s the only solution, but I’d need to know exactly what the problem is so I can calculate the correct counter-rotation to apply.
Note: the ~5 minute difference between accurate and displayed positions is equivalent to an angular solar position error of about a degree.