Datums

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The Earth’s surface is irregular and considerably more complicated than the perfect spheroid (or ‘ellipsoid’) that we usually assume it is. Therefore all maps project locations on the Earth’s surface onto a simplified datum. This datum is defined as a spheroid.

There are many dozens of datums in regular usage, because different datums do a better job of fitting different areas of the Earth’s surface. For example, the UK’s Ordnance Survey uses the OSGB36 datum, whilst the US usually uses the NAD27 or more recent NAD83 datum. Some datums (e.g. WGS84) have been developed for global use.

Maptitude’s country packs use different datums internally, depending on the area that they cover. For example, UK maps use ETRS89 for their layers.

Maptitude can convert (“shift”) between datums, but only when importing or exporting data.

The coordinate viewers within Maptitude remain fixed on your map’s datum. They do not take into account the datum implied by your selected coordinate system (if it has one). For example, if you select the “British National Grid” (BNG), the displayed coordinates/etc will use the correct BNG projection, origin, and scale; but they do not perform any datum transformation. If your base map uses the ETRS89 datum, then they are computed using the ETRS89 datum. Hence Maptitude’s British National Grid is technically not the British National Grid, unless you have explicitly created a base map that uses the OSGB36 datum. The observable error will vary but it is typically 50-100 metres in magnitude.

This issue also affects the API. If you wish to plot data points then you must use the base map’s datum, and not the datum implied by the coordinate system.

Maptitude can transform between datums when importing or exporting data, but you need to explicitly request the datum transformation. For example, MapInfo MIF files have the ability to include datum information with their coordinate system. Maptitude will read the coordinate system correctly, but ignore the datum. So when importing the MIF file, you need to press the Coordinates button on the import dialog box:

The Maptitude Import MapInfo Interchange File (MIF) dialog box. Press the “Coordinates” button explicitly set the datum shift

This displays the Import Coordinates dialog box:

The Import Coordinates dialog box set to treat the input coordinates as British National Grid with an explicit datum transformation from OSGB36 to ETRS89.

Note that this dialog box lets you override all elements of the coordinate system. The final Datum Conversion box lists the various datum conversions that are available. Here we are trying to plot true BNG coordinates on a Maptitude UK basemap, so we select Great Britain: OSGB36 to ETRS89. We also explicitly set the coordinate system to British National Grid because this will override the MapInfo header. The BNG includes its own scale, so keep the linear transformation set to the default identity.

If you wish to set the entire basemap to use a different datum, you will need to export your layers with the new datum shift, and then re-import them into a new map. Maptitude does not keep a record of what the current datum is, so this will work perfectly well. The important thing is that all new data is in (or is transformed into) the same datum as the existing map layers.

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