The Basemap Gallery widget sends an export request instead of requesting for tiles when used with a cached image service.
Last Published: January 11, 2016ArcGIS AppStudio
Bug ID Number
December 17, 2015
April 27, 2022
Operating System Version
7.0 64 Bit
Will Not Be Addressed
The development team has considered the issue or request and concluded it will not be addressed. The issue's Additional Information section may contain further explanation.
I was not able to see this with the attached app, I decided to swap out the Toronto ImageServer with that of the Delorme basemap service, http://server.arcgisonline.com/ArcGIS/rest/services/Specialty/DeLorme_World_Base_Map/MapServer
each time I toggled between this and the other basemaps, it never used the export function.
With this said, There are a couple of checks that are performed:
Does it have the same spatial reference?
Unless one of the layers only has one zoom level, are there at least two zoom levels common to one another that have the same scale value? This scale value is normalized by the tile info's DPI.
If either of these were answered "no" than the top layer is added dynamically using export calls instead of cached tile requests.
I'm guessing that this may be what you were running against. We are in the process of getting this information documented properly but for now this is the best info I can provide.