5. Conversion of data > 5.2 Vector maps
5.2.2 Import Vector Map from Ghana Grid (feet)
Level of expertise required for this Chapter: Intermediate; specifically for LUPMIS @ TCPD
The Survey Department holds some vector maps (also called ‘line maps’) in Ghana Grid reference, i.e. Ghana Grid TM projection, Accra datum, War Office ellipsoid, in feet.
Research work is currently undertaken about the conversion parameters (5-parameters or 7-parameters method, all fixed empirically). See Annex 8 and also Annex 1.1.7 for the procedure to set up projections at Map Maker). These conversion parameters are necessary to convert existing maps to UTM (WGS 84) – not for new maps and surveys.
Find below the process to convert maps from Ghana Grid in feet. (Very similar process to convert from meters is explained in Chapter 5.2.3).
1. If the file is not in SHP format, convert it to SHP format. For example, if it is in E00 format, see Chapter 5.4.2 and Annex 14.2.
2. Because the map is in feet, turn the conversion on for feet:
Main menu > File > System set up > Preferences > Set up window: Preferences > Miscellaneous > Tick ‘Assume shape files in feet’
3. Convert to DRA format and to UTM projection simultaneously:
Main menu > Utilities > Latitude and Longitude utilities > XY file to another projection > Vector > Choose source map projection window: Ghana Grid (if not available, define it, as explained at Annex 1.1.7) > OK > Select SHP as file type (no other import format possible) > Select folder and file > Choose target map projection window: UTM 30 > Select DRA as file type > Specify new file name > Save > OK
4. Don’t forget to turn the feet conversion off:
Main menu > File > System set up > Preferences > Set up window: Preferences > Miscellaneous > Box ‘Assume shape files in feet’ must be un-checked
5. Quality-check and see, if the converted data fit to other correct data, in particular GPS-tracked data (e.g. roads) or georeferenced photos.