PNG and PGW

[davidsrsb]

It would be very convenient to generate a .pgw world file when a .png plot file is made.
I have to do this manually to import as a raster file into QGIS and the details of the .pgw keep changing as plot radius is changed.

[der-stefan]

Are .pgw files a standard file for georeferencing raster graphics? I can't find any information on this file format.
In my own application I write a custom JSON file. Would that also be fine for you?

Edit: For documentation, here is an elegant way to load PGW files in Leaflet: https://github.com/RedfishGroup/LeafletElementOverlay

[dBitech]

.pgw files are also known as "World files" and are a 6 line textual sidecar file describing the extents, and the pixel size of the raster. see https://en.wikipedia.org/wiki/World_file for details

[der-stefan]

Ok, the only problem is that the map projection is not given in the world file. But it is no problem to add a world file output along the PNG.

[davidsrsb]

World files exist for most raster formats that don't have built in geo-referencing (ie geotiff)
pgw is the partner of png and is the favourite, as it is the smallest file that doesn't mangle text.
QGIS expect a pgw file, no concept of JSON.

This is a typical pgw file in Malaysia
0.0008333333
0.0000000000
0.0000000000
-0.0008333333
101.0000001211
3.9999999758

It gets more complicated away from the equator as pixel x and y size are no longer equal

[davidsrsb]

My example is using decimal degrees. This is convenient with SRTM3 terrain data
The Wikipedia example is using UTM

[der-stefan]

Georeferenced TIF files ("GeoTIFF") can be directly loaded into QGIS. This functionality is now implemented in the "json" branch and probably merged into the refactor/main/2.0 branch. Are .pgw files still important, as we now have .geo, .json, .tif and .kml for georeferencing? Can the issue be closed?

[der-stefan]

By the way: We can also write meta data to PNGs! That's not too complicated with libpng... Up to now, there is no standard to georeference PNGs with the help of meta data. Would be interesting to define a new standard!

Even a complete JSON string could be put into the PNG, as there is nearly no length limitation of "tEXt" meta contents. That opens up a new playground...