It is important to understand the difference between "defining" the coordinate system of a particular set of data, and "transforming" between coordinate systems. The Geosoft coordinate system tool is used to define the coordinate system of a set of data. When we display different sets of located data together on a map the locations of the data must be transformed from their defined coordinate system, to the coordinate system of the map. If both the data and the map have the same coordinate system, no transformation is required.
Data sets defined on the same datum but with different projection methods only need to be transformed across projection methods. For example, UTM coordinates on the NAD-27 datum can be placed on a Lambert Conformal projection map also on NAD-27 based only on the parameters of the UTM and Lambert Conformal projections.
However, when two sets of data are defined on different datums, we must also transform the data across datums. To do this requires the parameters defined by the "local datum transform". In Geosoft, the local datum transform provides the information necessary to reference the datum to WGS-84. The local datum transform of the destination coordinate system is then used to convert from WGS-84 to the required system. This will all happen seamlessly ("on-the-fly" if you like), provided the local datum transform is defined for both the original and target coordinate systems.
Choosing the correct local datum transform can be a challenge as it requires more advanced knowledge of the datums in use. Geosoft attempts to simplify this by choosing the most likely, or general local datum transform based on the datum that has been selected. However, to do this requires that the datum is fairly well-known and has one or more local datum transforms defined either in the EPSG reference tables (which we use as a standard), or have been historically accumulated in Geosoft software over the past 20+ years. There are more than 800 known and defined local datum transforms in our tables. If you are transforming data across datums, we recommend you verify that the default local datum transform is appropriate for the specific data you are working with. If you ever change the local datum transform for a specific datum, Geosoft remembers that this is your most likely default local datum transform should you choose that datum again.
When I use the "Copy From" tool in the Coordinate System dialog, and select an ESRI coordinate system, why do some systems carry a local datum transform while others do not?
In the case of ESRI coordinate systems, ESRI also supports most EPSG defined datums, and for those systems that also have EPSG defined local datum transforms, we are able to pick up the default local datum transform automatically. However, ESRI also has a number of datums that are not defined by EPSG, and for which we do not have a mapping in the Geosoft tables. In these cases, the Local datum transform will be blank.
NOTE: You should not attempt to display data from different datums together unless you have a defined local datum transform. In fact Geosoft will warn you if you attempt to do so.
If you do find yourself in this situation, it is most often because the datum defined in ESRI is in fact using a name different from the EPSG name. With some awareness of the datums in use in your area of study, you can often determine what the most appropriate datum should have been based on the defined ellipsoid. If you can determine an EPSG-compliant datum, you can change the datum. The ellipsoid parameters should not change. With an EPSG datum, you will generally have a default local datum transform, or you will be presented with a list of options. Be careful - if you get the datum wrong, the locations of your data will be wrong! When in doubt, you should always attempt to find an independent way to confirm that you have chosen the correct datum and local datum transform.
In summary:
The local datum transform definition is only required to transform between two different datums.
Where ESRI projection methods are imported using "Copy From", and the local datum transform is blank, this is likely a datum that is not recognized in EPSG or Geosoft. If you do not require transformations across datums, this is not a problem.
If you do require transformations across datums, you should see if the datum is in fact miss-named and might be one of the EPSG or Geosoft supported datums. This can be done with some investigation of the history of the data using the ellipsoid information as a guide.
The Geosoft default Local Datum Transform that is provided (when selecting datum\projection method combinations in the Coordinate System dialog parameters) is not necessarily the best choice for your area of interest. You should select an appropriate transform from the drop-down field.