Each quadrangle database contains UTM grid co-ordinates based on the Bessel 1841 map datum. The southern Sumatra data covers two UTM zones, namely UTM zone 47 (Padang, Solok, Painan and Sungaipenuh) UTM zone 48 (remaining Southern Sumatra quadrangles). The northern Sumatra data comes from three UTM zones: zone 46 (part of Banda Aceh and Calang; zone 48 (Siak Sri Indrapura); and zone 47 (remaining quadrangles).
All the quadrangle databases from the island of Sumatra have been combined to give a single database using longitude/latitude grid co-ordinates (file SUM98.XLS). The process of combining data from different zones and the different data sets of N and S Sumatra is outlined below. The S Sumatra co-ordinates are generally considered to be more accurate and are based on the 1:250,000 AMS topographic maps. Hardcopy drainage basemaps for the S Sumatra data are available from DMR in Bandung as the single element proportional symbol maps at 1:250,000 scale. The N Sumatra data is not so accurate and originates from old Dutch topographic maps. However, drainage bases on which these locations are based are available from DMR as hardcopy, 1:250,000 scale maps.
The combined database on the CD-ROM for southern Sumatra (SSUM95.XLS) presents all the data on the basis of UTM zone 48 co-ordinates. This is really a non-linear correction but on a regional scale near the equator it is acceptable to make a correction by subtracting 667,652 from the EASTING field in the UTM zone 47.
In order to create a single database for N Sumatra the UTM 47 co-ordinates are used as most of the data is from this zone. To achieve this, subtract 665091 from the Eastings x 10 of UTM 46 and add 667854 to the Easting x 10 values of UTM 48. The Easting is multiplied by a factor of 10 so as to bring the co-ordinates to be 1 grid unit equivalent to 1 metre and consistent with the Southern Sumatra databases.
In order to combine the N and S Sumatra data a correction also has to be made to the Northing. The equator is UTM Northing 0 for data from the northern hemisphere but UTM Northing 10,000,000 for data from the southern hemisphere. Therefore the 10m has to be added or subtracted according to which hemisphere you are standardising on.