If you’re using a custom image as the base layer, you can use this field (in conjunction with Top-most Coordinate, Left-most Coordinate, and Right-most Coordinate) to align your image with specific coordinates, so the nodes are displayed in your intended locations.

Enter a latitude in DD (decimal degrees) notation that coincides with the bottom border of the image. The default value is -85, which will place the bottom border of the image on the 85th parallel south.