Thread: GPS co-ordinates

Originally Posted by Cyclops

Technically speaking NZTM & WGS are very different. Ask your friendly surveyor - I'm not a surveyor.

Most coordinate systems use different underlying geoid systems.
The geoid is the system of mapping the coordinate system on to the physical earth.

Over a short distance different coordinate systems may be able to be made to match.

To add an extra ingredient to NZ's coordinate systems NZ's ground is moving at different rates.
Corrections should - in theory - be applied to a coordinate system over time to allow for the fact that NZ is moving and is moving in different directions at different rates.
That said it doesn't matter if your desired accuracy is +/-10 metres or more.

After the Canterbury earthquake systems a method of correction had to be created as the ground - including surveying markers - had often moved by several meters, thus making property boundaries incorrect.

Mate, thank you for this comment. My training on this as a user of navigation stuff follows like thus:

Geodetic datum is 'simply' the model used which represents the shape of the planet and locates all the features on the planet that the GPS or chartplotter shows you. If you are using the wrong datum different to that which the chart or map is drawn with the location in which you are shown on the chart or map won't tally correctly with the locations of the objects shown i.e., if you are driving a boat you might find yourself parked on solid ground thinking all was right in the world!

There are two that are in common use at the moment for us, WGS84 and GRS80 which at the equator are roughly speaking the same. There is a difference as you get closer to the poles which is due to the variations between the two models. WGS84 is the standard most chartplotters run by default, but it's not considered really accurate for NZ as the tectonic plates in NZ are moving by a minimum of 5cm a year. This requires fairly constant adjustments which isn't the easiest when the modelling is global - this is why NZ maps etc (LINZ) uses NZGD projection based on the GRS80 datum which is long story short set up to move with the features and doesn't go through a process of increasing errors followed by an update to the datum.

Doesn't really matter which datum you use provided that the data is up to date (and a lot of chartplotters aren't updated) and the format in which you are quoting the positions is the same as the info you are referencing (i.e. if you are quoting positions off Google Earth that are WGS84 datum and positions quoted in degree/decimals that the chartplotter or GPS that you are planning to use them with the GPS is also set to WGS84 and the same position format). For most of our navigation, the errors will be less than a meter or so which is within the level of error of the systems... LINZ website has a lot more info on the datums used in NZ and why we didn't end up with WGS84 datum that the yanks promote (it's boffin level technical).

The format in which positions are quoted in in NZ can be one of several different formats - degrees minutes seconds, degrees minutes decimal minutes, degrees decimal degrees or numerical. As long as you are quoting the position in the right format and the right datum, the map or the device will correctly interpret it. If you don't get it right, nothing will make sense and your positions will be wrong. Where it can fall over is with some systems that record positions in their own format - you then need to convert that to something that matches what you need to input the position into and that can be a pain in the arse.