There are several things that can go wrong when you’re using mapping data. One of these is the various assumptions made about the CRS (coordinate reference system): seismic and wells are surveyed in certain CRS’s and the use of incorrect parameters for the datums or spheroids used in these CRS’s can have a marked bearing on “where” in the world each of these items is actually located. So much so that a well could be drilled several hundred metres away from where it is “supposed” to be drilled – potentially missing the target completely. When you are dealing with very expensive offshore wells this can certainly affect a company’s bottom line, particularly if this information is stored in a central database which is then presumed to be correct. The potential for error can be enormous!

A lot of the issues around CRS’s involve dealing with the area’s local geodetic datum. A geodetic datum defines the way that latitudes and longitudes are interpreted such as the starting longitude (usually Greenwich), the spacing, the shape of the ellipsoid, and the origin.

A lot of people ignore these datum issues, usually because “it doesn’t matter (yet)” or because they don’t have the time to investigate them.

As an example, in Australia, there are 3 main datums in use: AGD66, AGD84 and the latest one – GDA94. Each of these datums is supposed to reference particular spheroids – AGD datums use ANS (Australian National Spheroid) and GDA uses GRS1980.  However, an issue that often occurred in pre-1970 wells and surveys, was the use of an older spheroid – the Clarke 1858 Spheroid. This spheroid has no nice or unique solution in the way it transforms to WGS84. So many users, therefore, supplied their older data with an imprecise datum transform applied which can lead to incorrect information being stored in their databases and then used with the assumption it is correct.

datum_editor

Petrosys’ datum editor screen showing the AGD84 datum and its associated parameters for spheroid (ANS) and prime meridian – data is based on the EPSG database.

A good data management technique is to check that the information is correct before loading data into any database or storage area. Data managers and users alike should always check the data being loaded into a database or storage area for correctness. This method should be recorded so that other users using the information at a later stage have enough detail to work out if, or where, anything went wrong. Surprisingly, the “unnecessary” re-checking of data can take considerable time so it is better to check once, check at the earliest possible point in the process and document the checking process.

For the Clarke 1858 Spheroid a lot of users chose to use the AGD66 (ANS) information instead.  Applying this AGD66 assumption to previously transformed coordinates will mean the actual location is different by a distance of about 160m. So, in Australia, it might be a good idea to consider supplying data with an inexact Clarke Spheroid datum transform (possibly +/- 10m or maybe +/-20m) rather than using a completely incorrect transform (incorrect by about 160m) for pre-circa 1970 data. For further information on datums in this area please check: www.ga.gov.au/scientific-topics/positioning-navigation/geodesy/geodetic-datums/historical-datums-of-australia/australian-national-grid

Petrosys software handles these transforms correctly using the latest EPSG transforms available (check their website www.epsg.org for details). In the Australian example above, we transform data to a WGS84 datum, then to GDA94 (or previous AGD datums) using the correct parameters as supplied by EPSG.

crs_configuration

The CRS configuration screen showing source datum>target CRS and the transformations that are available for selection.

Inevitably, there will be similar issues when using older datums in other parts of the world, too. It is always a good idea to compare your locations with other independent datasets, like Google Earth aerial photos, etc.

There are quite a few places from which coordinates can be sourced. For example, when dealing with well information the well completion report usually has coordinates listed in several places – like the well summary, or the surveyor’s report, or in the directional survey report if one was done. Post-well location surveying (perhaps done years after the well was drilled and perhaps done offshore with a magnetometer or other remote sensing survey method) can also give you a better set of coordinates.

So, for users of coordinates (and, really, that is all of us), having a solid database or storage area to record the correct coordinates can help you manage these locations and any checking or transforms done on them. This should lead to better selections for well locations and any other future work and, hopefully, will avoid some of the pitfalls that incorrect locations can lead to.