EPSG:9816

Note: These formulas have been transcribed from EPSG Guidance Note #7-2. Users are encouraged to use that document rather than the text which follows as reference because limitations in the transcription will be avoided.

This grid is used as the basis for mineral leasing in Tunsia.  Lease areas are approximately 2 x 2 km or 400 hectares.  The southwest corner of these blocks is defined through a six figure grid reference where the first three digits are an easting in kilometres and the last three digits are a northing.  The origin of the grid is Djebel Kebar at 38.81973gN, 7.83445gE of Paris at which the grid reference is 270582. The latitudes and longitudes for block corners at 2 km intervals are tabulated in a mining decree dated 1st January 1953.  From this tabulation in which geographical coordinates are given to 5 decimal places it can be seen that:
a)  the minimum easting is 94 km, on which the longitude is 5.68989 grads east of Paris.
b)  the maximum easting is 490 km, on which the longitude is 10.51515 grads east of Paris.
c)  each 2 km grid easting interval equals 0.02437 grads.
d)  the minimum northing is 40 km, on which the latitude is 33.39 grads.
e)  the maximum northing is 860 km, on which the latitude is 41.6039 grads.
f)  between 40 km N and 360 km N, each 2 km grid northing interval equals 0.02004 grads.
g)  between 360 km N and 860 km N, each 2 km grid northing interval equals 0.02003 grads.

From the above the latitude of N=360 can be determined to be 36.5964gN.

Formulae are:

Grads from Paris

Lat = LatO + [(N - Nf) * A] 
where Lat and LatO are in grads, N and Nf are in kilometres and A = 0.010015 if N > Nf, else A = 0.01002.

Lon = LonO + [(E - Ef) * 0.012185] 
where Lon and LonO are in in grads relative to the Paris meridian and E and Ef are in kilometres.

The reverse formulae are:

E = Ef + [(Lon - LonO) / 0.012185]
N = Nf + [(Lat - LatO) / A] 
where A = 0.010015 if Lat>LatO else A = 0.01002.

Degrees from Greenwich.

Modern practice in Tunisia is to quote latitude and longitude in degrees with longitudes referenced to the Greenwich meridian.  The formulae required in addition to the above are:

Lat (degrees) =  (Latg * 0.9) where Latg is in grads.
LonGreenwich (degrees) = [(LonParis + 2.5969213) * 0.9] where LonParis is in grads.

Lat (grads) =  (Latd / 0.9) where Latd is in decimal degrees.
LonParis (grads) = [(LonGreenwich / 0.9) - 2.5969213)] where LonGreenwich is in decimal degrees.