|Figure 1 - finding the datum manager|
Each datum available for selection is actually a class of datums, i.e. each selectable datum branches out into several datums. The common
denominator is the ellipsoid - it is the transformation to WGS84 that differs between the variants of the datums. Each variant encapsulates
a parameter set for conversion to WGS84, and a transformation is required for a conversion to take place. The variant that is used when
converting is the one set to default in a particular region. You can change this by clicking the Default in [region] button in the
variant field of the datum manager. See figure 2.
|Figure 2 - selecting a datum variant|
Some datums in the database, such as RT38 (Stockholm), does not have any transformation to WGS84 associated with them. For this reason,
you cannot transform coordinates given in RT38 (Stockholm) - or any of the grids based on RT38 (Stockholm) - to another datum. To enable
transformations, you need to add or modify a datum variant and mark it enabled for transformations. To do that, check "Datum transformations
enabled" (see figure 3) and fill out the parameters. Once you have done this, you can use the datum as any other datum.
|Figure 3 - enabling transformations on a datum|
Every datum available in the database and the datums you create can be exported so that it can be imported later or on another machine. To export a datum, simply select it and click export. If you select an existing file you are given the option to merge the datum into the file, which means that you can create a database containing several datums and/or grids and/or coordinate formats. See figure 4.
To import a datum you simply click import and select the file that contains the datums. The datums will be imported and sorted according to the regions they are associated with in the file being imported. If a datum being imported already exists in the database you are given the option to overwrite the existing datum if it differs from the one being imported.
|Figure 4 - exporting a datum|
If you wish your datum(s) to be accessible in another region, just drag the datum(s) from the datum list and drop it/them on the region in which it should be accessible. This will associate the datum(s) with a new region, i.e. if you change the datum(s) in one region, it/they will change in the other as well.Can't find your datum?
If you don't know where your datum is, you can always search for it in the find-box (see figure 5). To search for a datum, simple enter
part of its name, ESRI number, reference meridian etc to search through the database. Note that when you search for a datum, you don't
associate the selection with a region. This means that if you remove a datum you have found from a search, it will be completely removed
from the database.
|Figure 5 - searching the database for a datum|