Article: 13788 of sci.geo.satellite-nav Message-ID: <3C5F1873.190EE713@alphalink.com.au> Date: Tue, 05 Feb 2002 10:25:39 +1100 From: Graham X-Mailer: Mozilla 4.78 [en] (Win98; U) X-Accept-Language: en MIME-Version: 1.0 Newsgroups: sci.geo.satellite-nav To: Maria de =?iso-8859-1?Q?F=E1tima?= Rodrigues de Carvalho Subject: Re: Newb question References: <3c5f0083@213.30.5.12> Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: 8bit NNTP-Posting-Host: d52-ds11-mel.alphalink.com.au X-Original-NNTP-Posting-Host: d52-ds11-mel.alphalink.com.au X-Trace: news.alphalink.com.au 1012864806 d52-ds11-mel.alphalink.com.au (5 Feb 2002 10:20:06 +1100) Organization: Client of Alphalink Australia Pty. Ltd. Lines: 55 X-Admin: admin@alphalink.com.au X-No-Archive: Yes Path: news.meer.net!sea-read.news.verio.net!dfw-artgen.news.verio.net!dfw-peer.news.verio.net!news.verio.net!newsfeed.mathworks.com!news.maxwell.syr.edu!news.mel.connect.com.au!news.netspace.net.au!news.alphalink.com.au!news.alphalink.com.au Xref: archive.mv.meer.net sci.geo.satellite-nav:13788 "Maria de Fátima Rodrigues de Carvalho" wrote: > What is a Datum? The year is 1927. I want to map North America, using the latest and greatest equipment. I start in the middle of a paddock in Kansas and try to decide where I am. By using astronomical techniques (what else can I use if I have no GPS and no pre existing accurate survey mark?) I decide where I am. My accuracy is within perhaps half a mile, that is to say, I know how far I am from the Greenwich meridian etc to within half a mile. I now map Kansas very accurately, using chains, theodolites, etc. My accuracy is within perhaps 10 feet total error over all of Kansas. I now have a map of Kansas which is internally consistent and accurate. The year is now 2001. You come along with your newfangled GPS. You check the location of a point in Kansas and find the GPS says it's 1/2 mile west of where the 1927 maps says it should be. How odd. You check the location of another point in Kansas and find the GPS says it's also 1/2 mile west (and perhaps 4 feet north) of where the 1927 map says it should be. Curiouser and curiouser says Alice. You check the location of a third point in Kansas and find the GPS says it's also 1/2 mile west (and 3 feet south) of where the 1927 map says it should be. You look down at Toto and wonder if you are still in Kansas. So you go find the starting point in the middle of THAT paddock in Kansas and check it. And of course you find that the starting point was out half a mile west from the very beginning. So you persuade your GPS to display positions using the Kansas 1927 datum (more correctly, the NAD27). It simply shifts where it says everything is east 1/2 a mile to match the maps which were made using the NAD27. Now for a disclaimer: The difference between datums is rarely as large as 1/2 a mile! I have no idea what the difference between NAD27 and WGS84 is in Kansas, in either direction or magnitude! But now you see where the need for datums has come from. > What's the difference between Datum Lisbon 1954 and Datum Lisbon 1970? Probably something between nothing and a few hundred metres.