How to convert attribute table fields in WGS84 meters to fields in decimal degrees in QGIS

Question

I have fields where location data is in X- and Y columns in WGS84 meter-format. How can I convert these fields or create new fields with decimal degrees? Vector->Geometry Tools-> Export/Add geometry columns creates duplicate fields with the same meter-format. Similarly using field calculator with $x- and $y functions creates also fields with meter-formats.

Answer 1

I may be misinterpreting the question, but WGS84 is a geographic coordinate system, utilizing the WGS84 ellipsoid, its coordinate space is measured as lat long pairs and not meters. See unit of measurement here or here. As such WGS 84 is not represented as meters, see discussion here, here or here (comments). In short, WGS84 uses angular measurements to represent the locations within a three dimensional space, as a metered grid doesn't envelope the earth very well. WGS84 is always projected when displayed in GIS software (without changing the underlying data), it is projected to convert it from a 3 dimensional representation of the earth to a 2 dimensional.

Your data, if measured in meters, is projected. The WGS84 ellipsoid may be used as part of the basis of a projection, such as with UTM or WGS84 Antarctic Polar Stereographic. The projection you have and its parameters are critical to understanding how you determine the position of a point in degrees, as a point will essentially have to be unprojected to get its latitude and longitude.

Luckily this is relatively easy in GIS software.

In QGis you can change the coordinate reference system of your layer to WGS 84 (EPSG:4326) - which it could be already with the data coming from a different source or previous CRS - and then use the field calculator to calculate the geometry that you are looking for (assuming that your fields in meters represent something that can be calculated by the field calculator). This also requires your existing data to have a defined projection. If needed you can convert back after you have added the new data.

In Arc, the process is largely the same, using the "project" tool to reproject/unproject the data.

If your data layer does not have a defined projection, you will need to find it. If your data layer fields that are already in meters are not something easily calculated from the field calculator in qGIS, then it might get a little more involved (creating a layer from those fields, changing the CRS of that layer, calculating the fields in degrees...).