How to add a custom base map
If you have data specific to a certain region, you probably don't want to display it on a world map.
To add a custom base map to your survey visualization,
The map needs to be in equirectangular projection. If you don't already have an appropriate map, you can create the map as you want it in the projection map template (potentially uploading custom GeoJSON as required), removing all the margins, legend, headings, search box etc. When you are happy, download the image as a PNG.
This is an equirectangular projection of Brasil – click here to duplicate this example
Next, in the "Map" settings, select "Custom" and upload the image.
Next, you need to set the bounding box for the latitudes and longitudes of the map, so that your locations will display at the correct position on the map. If you don't know the values for the bounding box, you might be able to find them here.
x1 = Minimum longitude (x1)
x2 = Maximum longitude (x2)
y1 = Minimum latitutde (y1)
y2 = Maximum latitude (y2)