Merge pull request #442 from opengisch/gnss_swiss

add an example how to get swiss altitude

documentation/how-to/gnss.en.md

@@ -84,6 +84,10 @@ For example:
 For the transformation from ETRS89 (reference ellipsoid GPS) to NAP (Dutch) users can download
 the file [nlgeo2018.gtx from NSGI](https://www.nsgi.nl/rdnaptrans) and put it in the directory.
 
+To obtain precise altitude data for Cadastral Surveying in Switzerland, users can access the file correction of the vertical grid shift through [Geoid OGD from Swisstopo](https://cms.geo.admin.ch/ogd/geodesy/Geoid_OGD.zip).
+Following the download, users are advised to perform a conversion of the file labeled `chgeo04_htrans_lv95.agr` to `chgeo04_htrans_lv95.gtx`.
+The QGIS processing algorithm `gdal:translate` (convert format) can be used for that.
+
 ## Usage
 :material-tablet: Fieldwork
 
@@ -146,6 +150,7 @@ The logs will be placed in *Android/data/ch.opengis.qfield/files/QField/logs*.
 !!! note
     Be aware that if the log is always turned on, it will fill up all the storage.
 
+
 ## Mock location
 :material-tablet: Fieldwork