You quite often see campus layouts, or site plans or some other drawn data that are an approximation of a real place. It would be good to be able to put markers on such a layout driven off Sheet data. I searched for a random map of this nature and found this one, which is a proposed (or actual) development somewhere in Utah.
The next step was to find the location on maps, and get the approximate co-ordinates of the south west and north east limits represented by the plan. That allowed me to describe the plan like this, where I'm hosting the image on Google Drive, and when I place it on top of a map. I want it to be somewhat transparent so I'm giving it an opacity of 60%.
In addition to overlaying this plan on maps, I also wanted to plot a few spots - in this case representing building lots, using regular and custom maps markers. The add-on in action looks like this.
or even ...
The data for the points is read from the associated sheet and looks like this. Not specifying a marker will use the default one, otherwise you'll get whatever image it links to. The map is automatically scaled to the overlay image, and centered around it.
This is pretty straightforward, although you would need to clean it up a bit for a real add-on.
You'd modify the settings here to describe your plan overlay.
Let me know if you build something nice with this. You can try it out here, but try not to screw up the test data for others...
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