The Mudcat Café TM
Thread #103645   Message #2114469
Posted By: Rowan
30-Jul-07 - 01:52 AM
Thread Name: GPS systems for cars
Subject: RE: GPS systems for cars
This might be either stuff you all know or stuff you have no need to know but you should make sure that your GPS is using the currrent "source" data for your region. GPSs display their "Location", which is calculated by reference to at least 4 satellites. Satellites orbit a centre of gravity (not necesarily the centre of the earth) and the Location the GPS displays is on a particular spheroid. Different regions have their spheroid calculated differently because the earth isn't a true sphere and the difference varies all over the earth's surface.

Maps are constructed according to a mathematical model of the spheroid that applies to a particular region and the notional 'centre' of the earth for that bit of map is called the geoid; the centre of gravity for the satellites has no particularly concrete relationship to any geoid.

Most people work out where they are, where they want to be and how to get there by using maps of the ground rather than by abstruse notions of centres of gravity.

Some cautionary tales to help explain;
Early in the use of GPS instruments some people were trying to use them to navigate in SW Qld where the maps had been constructed with the geoid quite some distance from the sateliites' version of the centre of the earth (let alone the centre of gravity) and found that the maps were the only reliable source of info they could apply.

A fellow was given a GPS to go and plot some locations and did so for several weeks, recording them on the forms required. Some weeks after he left the employ of the organisation other workers tried to relocate his plottings and couldn't get within cooee of his locations, which were in western NSW. He had been using the GPS not realising the GPS had been used in Pakistan and the "source" in the instrument was still set for Pakistan, which has a quite different geoid. Fortunately they were able to reset "to Pakistyan" and relocate his plottings.

Take home message? Make sure the thing has been properly set up for the area you intend using it.

Cheers, Rowan