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Mitsubishi 4WD Owners Club of Qld (Inc) |
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GPS - Part 6 |
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GPS-101 Part 6 (More on Maps) Before I get into this month’s topics of map projections and datum and coordinate systems, I just thought I’d give you a bit of an update on the GPS Satellite systems themselves. The GPS system most commonly used in Australia is the USA Department of Defence, NAVSTAR system. The other constellation of GPS satellites called GLONASS is owned and operated by the Russians. Well there are going to be a couple more constellations of GPS satellites up there over the next decade or so. The Chinese are planning to launch their Compass system and the European Union’s Galileo system is planned to be operational by about 2014. The Galileo system will provide an Open Service Signal from which receivers will achieve an accuracy of <4 m horizontally and <8 m vertically. It is expected that civilian receivers will process both the NAVSTAR and the Galileo signals, for maximum coverage. The US and the EU have agreed on a common signal for use by their satellite navigation systems to provide more accurate images and information. The NAVSTAR system will also be getting an upgrade over the next decade, with the launch of its GPS III satellites. The bottom line for us users is that the systems will continue to become more accurate and provide better coverage than is available now. Last month, I started to talk about maps and the use of moving maps. The maps that are generally of most use to us 4WDers are the raster maps used in software such as OziExplorer. However, there are some concepts that you need to understand about using a GPS with a map and also calibrating maps. These are Datum and Coordinate systems, and Map Projections. Firstly, what is a map? It’s a diagram that is a representation of something else. The maps that we are most interested in are maps that represent parts of the earth’s surface and the natural and man-made things that exist on it. These maps tend to be flat, because it’s convenient to have a flat map. The problem though, is that to represent something that is not flat, in a flat representation without introducing any distortion, is simply not possible. So what we do is distort the representation of the earth’s surface in a predictable way by a known mathematical process. Once we know how the map has been distorted, we can interpret the representation. This distortion is called a projection. The other things that we need, in order to be able to interpret where we are, is a datum and a coordinate system. The datum provides a point of reference, and the coordinate system describes in three dimensions, where you are, or where points on the map are, in relation to the datum. Now, it’s really important that when you’re dealing with maps and GPSs that you use that same datum and coordinate system on both. The same applies when you’re relaying GPS coordinate information to somebody else. If you’re using a different datum or coordinate system, then you’re just not going to get it right. Here’s a simple example. Barnacle Bill says to Fearless Fred, “AAAAARRRRGGGH, I buried the treasure 150 paces toward the setting sun from the big pine tree near the cliff at Smuggler’s Cove.” Bill has probably got very little chance of finding the treasure. Firstly the datum is ambiguous: - Which Smuggler’s Cove is he talking about? There are lots of them. - Which was the biggest pine tree when Bill buried the treasure? Next, the coordinate system is lacking in information: - Bill is a tall man, but he has one wooden leg. So, how long is one of his paces? - The direction of the setting sun changes throughout the year. So what time of the year was it when he buried it? Imagine if Noah didn’t know how long a cubit was. If he and God weren’t using the same system, the Arc may never have been build big enough to accommodate all of the animals, or it could have been too big and not have been finished in time for the flood. Latitude and longitude have been used as a coordinate system since the mid 1600s. It is one of the simpler coordinate systems to understand. It describes the location on the earth’s surface using two angles. Latitude gives the location north or south of the equator. Lines of Latitude are the horizontal lines shown running east-to-west on maps. Latitude is an angular measurement in degrees from 0° at the Equator (the latitude datum) to 90° at the poles.
Longitude describes the location east or west of a north-south line called the Prime Meridian. Longitude is given as an angular measurement ranging from 0° at the Prime Meridian (longitude datum) to +180° eastward and −180° westward. Unlike latitude, which has the equator as a natural starting position, there is no natural starting position for longitude. Therefore, a reference meridian had to be chosen. While British cartographers had long used the Greenwich meridian in London, other references were used elsewhere, including: Rome, Copenhagen, Jerusalem, Saint Petersburg, Pisa, Paris, Philadelphia and Washington. In 1884, the International Meridian Conference adopted the Greenwich meridian as the universal prime meridian or zero point of longitude.
The angles of longitude and latitude are either represented as degrees and decimal fractions of a degree (e.g. 152.567432°) or degrees, minutes, and seconds (e.g. 123° 34’ 24”). There are 60 seconds in a minute and 60 minutes per degree. Sometimes you’ll also get degrees, minutes and decimal minutes (e.g. -23°45.6622’). So the coordinates for Brisbane would be -27.46°,153.02°. One of the things that you need to remember when you’re navigating is that parallels of Latitude are parallel, where as meridians of longitude are not. Let me give you a practical example. If you flew due south from Brisbane for 200km, then flew due west for 200km, then you flew due north for 200km, you’d need to fly 203km to get back to Brisbane. More often these days you’ll see coordinates for Brisbane that look like this: 56J, 503649E, 6962919N. What do these mean? These are UTM (Universal Transverse Mercator) coordinates. This is a projection mechanism for making the earth flat so that is can be represented on maps. I’ll talk more about UTM next month. Happy navigating. Greg Conlon |
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