modern GPS-receiver are able to use self-defined user grids or on the other hand it is possible to calibrate an existing grid on a map, which is not corresponding to a common projection like UTM-grid.
With your helpfull informations I could calibrate the green grid printed on the map of the Caldera scaled 1:5000.
In case that someone else want to use the Caldera map with a GPS receiver I will give you the information for setting up the device.
For example using Garmin receivers like the GEKO or the eTREX you have to go to MENU page then to SETUP. On the SETUP page then go to UNITS. Choose POSITION FRMT and then choose USER GRID. Then enter the following values:
LONGITUDE ORIGIN: E 034°35.000
FALSE E: 19000.0
FALSE N: -49789.0
then press SAVE.
the Point E 34°35´ N 1°08´ then will appear on the display :
0019000 -- East value
USR 0070000 -- North value
The "Round Top Hill" in the Center of the Caldera located at E 34°33´31,43´´ N 1° 08´19,93´´ will show the following coordinates on the display:
0016261 (East) -- 16 ist the number of the E grid line and 261
means 261 meters east of the line
USR 0076140 (North) -- 70 " N " 140
means 140 meters north of this line
With kind regards
Dr Harald Hoyer