How to fix the “Can’t warp” error

In some cases, while working with MapTiler Engine, you might come across the “can’t warp” error message. In this article, we’ll explain the possible reasons for such an error and show you what you can do to fix them.

The error

It might happen that, while trying to render a map, you’ll get the error that looks like this:

Below, you’ll find the two most common causes for this error and solutions to fix them.

Incompatible coordinate systems

If you used the “Assign visually” function from the “Geolocation” menu, you had to choose the ground control points (GCPs), to overlay your input image over the basemap. If you changed the input coordinate system after georeferencing the image, you might get the “can’t warp” error. The reason for it is that the GCPs are saved in the coordinate system that was chosen at the moment of opening the georeferencing tool and some coordinate systems are inherently incompatible. Take for example Web Mercator(EPSG:3857) and WGS84(EPSG:4326) - the former uses meters as the basic unit while the latter operates on degrees and both have completely different bounds. Therefore, if you chose the GCPs in the Mercator projection, and then switched to WGS84, the GCPs you picked will not make any sense in the new projection.

Solution

1. Change the setting in the “Geolocation” menu to the default one (or any other than “GCPs” for that matter).
2. Choose the input coordinate system you wish to use using the “Coordinates” menu.
3. Georeference the image once again using the visual georeferencing tool (“Geolocation” -> “Assign visually…“)

Geolocation out of the CRS bounds

Another common cause for the “can’t warp” error is setting the input image geolocation values out of the bounds of the chosen coordinate system. For example, the Web Mercator projection is defined between 85.06°S and 85.06°N. If you set any of the geolocation latitude values beyond this range, you’ll most probably encounter the error.

Solution

Change the geolocation settings (using the “Geolocation” menu) to be inside the bounds defined by the chosen coordinate system.

Usage tip

You actually don’t have to go through the whole wizard to learn that you made one of the mistakes mentioned above. If the set geolocation values are incorrect in the chosen coordinate system, you will not be able to preview the footprint of the image (the “VIEW FOOTPRINT” button at the bottom of the window will be disabled). It should look like this:

Conclusion

At this point, you should be able to handle the “can’t warp” error in most of the cases that it might happen to you. If it does, just double-check the combination of the “Geolocation”and “Coordinates” settings for possible errors.

Useful links

Coordinate reference systems
How to overlay an image over a map
Georeferencing
Georeferencer functionalities