Local park maps often show trails and other features that don’t show up on mapping apps... but the local maps can’t show your current location.
ploverlay makes it (relatively) easy to bring these two together. Given the URL of a map, and the latitude and longitude of four control points, ploverlay will show you where you are on the map.
(Plovers are (cute!) shore birds that can return to the same nesting location year after year. Hopefully, good map tools can help people find their way in the outdoors, too.)
After you clone the repo and cd into it, you'll need to start Jekyll:
bundle exec jekyll serve
For a new map, copy one of the examples under _maps/.
For each control point we need a geographic latitude-longitude and an image x-y.
For geographic coordinates in North America, the latitude will be positive and the longitude negative.
For images, the origin is in the upper right, with x increasing to the right, and y increasing downward.
A view_box also needs to be specified: Typically, your control points will be the corners of the image,
and you will use the full height and width of the image, but that is not a requirement.
Note that because we use four control points, a projective transformation is possible: This means that you could use an oblique aerial photograph, as long as the landscape below is relatively flat, and four control points can be precisely located in the image.