Rectified Map Not Displaying Properly in GPS App

[DonpK]

After rectifying a map, when I compare the control points shown in the Map Warper Control Point table, the longitude and latitude of the each point matches quite well with the OSM web version. However, when I load the geotiff version into my GPS tracking app, Avenza Maps, the coordinates of the control point locations are substantially in error.

I sent the file to Avenza and they said there are " issues with the georeferencing", not with their app.

Do you have any suggestions of how I can troubleshoot this problem? It could be a problem with the way I'm using Map Warper.

Don

[timwaters]

Hi, could you include the link to the map on mapwarper?

[DonpK]

Hi Tim, Here is the link to the Lancaster 1831 map: https://mapwarper.net/maps/44692

The map currently has only three control points, however, I tried earlier attempts with more points.

Thanks for your help

[timwaters]

Hmm, looks strange when warped:

Edits - ahh problem with the cropping mask. Looks ok now

[timwaters]

looks okay now. Give it a go now with the app

[DonpK]

Thanks for your help. The map looks good now in the Avenza Maps app. Also, I realized I was making a stupid mistake. The Avenza coords are in degrees decimal minutes and the Map Warper coords are decimal degrees. Once I did the conversion, things looked much better. Did I do something wrong in the cropping procedure? For what it's worth, here is a screenshot of the Avenza details on the map:

[DonpK]

Tim, could you explain what I'm doing wrong when cropping the map and how to undo it? It happened again with another map: https://mapwarper.net/maps/44895 The rectified map looked good until I tried to crop it, then it turned into a fuzzy blob.

[timwaters]

Hi Don, in this case it wasnt the crop. I deleted the cropping mask and added a basic one, sorry.

In this case there were 3 identical control points on top of each other, so I deleted the dupes and it looks okay now

I'll close the issue now :) Thanks