A guide to map grid references, latitude/longitude,
datums, GPS use and other basics of spatial data
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A guide to map grid references, latitude/longitude,
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Overview This website explains how to locate a place in Tasmania using numbers. There are two ways to do this. You can either state the grid reference of the place in the UTM system, or you can give the latitude and longitude ("lat/long") of the place. Both methods are covered in depth on these pages. Here's an example, a place in the centre of Tasmania: Location in words Summit of Mushroom Hill Grid reference DP777320 Lat/long 42°09'49"S 146°43'48"E How to get those numbers? If you were standing on the summit of Mushroom Hill with a GPS unit in your hand, you could simply set the GPS to record either the grid reference of your position or its lat/long. Alternatively, you could find Mushroom Hill on the 1:25 000 scale Waddamana map sheet produced by the Tasmanian government. You could then use the grid of UTM lines on the map to estimate the grid reference of the summit. How to do this is explained on the locations from maps page. You could estimate the lat/long of the summit from the map as well, but as you'll see, that's a lot harder to do. This website also explains the "fiddly bits" in locating with numbers. For example, the Mushroom Hill grid reference should properly be given as either DP777320 (AGD66) or DP778322 (GDA94), and the lat/long as either 42°09'49"S 146°43'48"E (AGD66) or 42°09'44"S 146°43'53"E (WGS84). The strange-looking codes in brackets ("AGD66", "GDA94" and "WGS84") are datums, which are discussed on the datums page. It's a good idea to specify which datum you're using when you give a location in numbers. Compare the figures and you'll see what I mean. There are several different ways to write the same grid reference. A GPS unit, for example, might show the Mushroom Hill summit as 55G 0477700 5332000 instead of DP777320. Lat/long's, too, can be given in different formats: 42°09'49"S = 42°09.82'S = 42.1636°S The different formats are described in detail on the UTM grid references and latitude/longitude pages. You can convert from one UTM or lat/long format to another using the information on those pages, or you can let the tools on the conversions page do it for you. Also on the conversions page is information about free software that interconverts UTM grid references and lat/long. King Island is a special part of Tasmania because it straddles two different UTM grid zones. If you're reporting grid references from King Island, be sure to read the page on the King Island problem. A very important issue in locating a place is error. Is your location just where you say it is? Plus or minus how much? How do you know? Finally, on the locations in words page you'll find a discussion of problems involved in using words to describe where a place is located. Good practice is to use both words and numbers.
This guide to spatial basics is hosted by the School of Geography and Environmental Studies at the University of Tasmania, but it's all my own work and any enquiries or feedback should be directed to me by email. The website started out in 2002 as a desktop reference in HTML for staff at the Queen Victoria Museum and Art Gallery (QVMAG) in Launceston. It was (and still is) aimed at helping map- and GPS-users understand more clearly what those numbers on the map margins and the GPS screen are really all about. Bob Mesibov
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