QUESTION: How to get WGS 84 coordinates from GeoTiff?

[jorrit-g]

Hi there,

I'm fairly new to the GIS domain so maybe i'm not understanding some things.

I'm reading a geotiff file with geotiff.js but now i don't see any method to convert a pixel to an actual WGS84 coordinate?

I do see the GeogTOWGS84GeoKey but I have no idea how to use this key to convert tiff pixels to coordinates.

Could anybody shed some light on the floats in GeogTOWGS84GeoKey?

Happy to provide make a PR for a image.toCoordinate() method (if that would be even possible?)

Much appreciated!

[constantinius]

Hi @jorrit-g

Currently, geotiff.js only provides rudimentary information about the internally used projection and location. This information is stored in some special tags called geo keys. The full list of them can be seen in the specification. Unfortunately the possibilities to encode geospatial referencing are quite diverse, so geotiff.js never included anything more than the most rudimentary form.

Also, geotiff.js will likely never support reprojection of coordinates and rasters, as this is a much more complicated field and out of scope of this project.

For starters, I suggest the following approach:

• If you know that the used projection is actually WGS84 (or the GeographicTypeGeoKey is 4326) then you can apply some math to project the the X/Y image coordinates to WGS84. The following is to be understood as pseudocode:

const origin = image.getOrigin();
const resolution = image.getResolution();

const lon = origin[0] + x * resolution[0];
const lat = origin[1] + y * resolution[1];

• When this is not the case, the problem is more tricky, as you first need to transform the image coordinates to the native projection and then reproject to WGS84. This should be doable with proj4js when you have the according projection information. If you are lucky they are simply set in ProjectedCSTypeGeoKey or GeographicTypeGeoKey and you can get the projections WKT representation from epsg.io. See here how it was done in the COG-Explorer.

I hope I could help you with your problem. Let me know if there is still some question unanswered.

[jorrit-g]

@constantinius thank you for your reply and help! I did have the projection info in my tif under ProjectedCSTypeGeoKey and was able to reproject with proj4js and epsg.io. Thanks again.