I just sorted my maps onto separate chips and grouped the ones that played nicely with each other. CA-NV-UT, ID-MT-WY, AZ-NM-CO, WA-N West-N Central & AK. The AZ and NV maps are on separate chips now so no worries. Kinda like having in-laws that don't get along. Thanks for all your input.
So there is no 'standard' to to trim at the border and not interfere with adjacent states? It seems that the CA topo, by the nature of CA running at a diagonal along its eastern border, doesn't interfere with the NV topo even though there is overlap all along that border since the segments are rectangles. So when we download and make a chip with multiple states we need to check all of the border areas? I don't know anything about making maps, I was just surprised that blank data sets can supersede data sets of another state, and that some maps play nice with border states, and some don't.
Maps downloaded from GPS depot for Nevada and Arizona show all appropriate data.
Sending data to my Garmin with MapSource, along the borders of states, I find that one states data will over write the other states data for the same sector and do so in such a manner that I can not control it. Map segments of one state may have blank areas over the border for the adjacent state's data.
Here is a specific example using the Arizona topo and the Nevada topo:
Nevada topo segments Topo K-7 and L-7 the Gold Butte areas west of the Arizona border, south of I-15, north of a branch of Lake Mead.
Arizona topo segment B-1 that overlaps Nevada L-7
The Nevada L-7 has all the topo data, the Arizona B-1 is blank for the Nevada section of the map segment. The Arizona segment always trumps the Nevada segment when these are transferred to the device so that Nevada data is replaced with the Arizona non data.
I see no way in MapSource to arrange segments or prioritize segments such that the Nevada data trumps the Arizona non data for that area.
There appears to be no way to nudge MapSource to use the Nevada segment and always applies the Arizona blank segment. Any ideas for a fix for this since there may be other blank areas in a stitched download with no data and I'd like to find that out before hitting the back roads.
I tried combining the western states and had similar results with border areas with lost data on the garmin device. I see detail on the individual maps in MapSource but they are not there on the garmin. I suspect that when the software transfers data and builds the .img file it may take what is a blank data on one map (because it exists in an adjacent state, other side of the border) and overlays the lack of data onto the complete data fields of the adjacent state. A stiching error between states. Just a guess. Since MapSource is built by Garmin for Garmin it isn't a surprise that it doesn't do a good job of stitching together independently produced maps. Clearly the problem is in the transfer process and .img file compilation. May need to do one chip one map to insure you have all of the state's data.