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199609 [2016/06/17 02:00]
vievems [Come to Cairns]
199609 [2016/06/17 02:29] (current)
vievems [GPS Units]
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 My name was mentioned in the May news in connection with GPS (Global Positioning System) units. I have gathered some information on these devices which I think will be of interest to members who have heard of these but have not had the time to look more deeply. I should mention that I have never used one of these devices so some of the following is based upon experiences reported by others. My name was mentioned in the May news in connection with GPS (Global Positioning System) units. I have gathered some information on these devices which I think will be of interest to members who have heard of these but have not had the time to look more deeply. I should mention that I have never used one of these devices so some of the following is based upon experiences reported by others.
  
-The units operate by receiving radio wave signals from satellites. The frequencies used are such that they are significantly affected by moisture. Some units are not useable in forests due to the attenuation of the signals by moisture in the leaves. It may be necessary to find a 'clearing in order to get a positive reading. Similarly, rain Or falling snow can effect the ability of the units to operate correctly. +The units operate by receiving radio wave signals from satellites. The frequencies used are such that they are significantly affected by moisture. Some units are not useable in forests due to the attenuation of the signals by moisture in the leaves. It may be necessary to find a clearing in order to get a positive reading. Similarly, rain or falling snow can effect the ability of the units to operate correctly. 
-Other objects may attenuate, reflect or otherwise impair the signals. ​for bushwalking this may be difficult or impossible to obtain a meaningful + 
-position reading if you are in a valley, gully, hut or similar situation.. +Other objects may attenuate, reflect or otherwise impair the signals. ​For bushwalking this may be difficult or impossible to obtain a meaningful position reading if you are in a valley, gully, hut or similar situation. 
-The current practical accuracy of GPS for bushwalking under good conditions is about 100 'metres horizontally,​ and much worse vertically. There are more accurate alternatives such as differential UPS (of which there are two basic kinds) and GLONASS (the Russian + 
-system which is roughly similar to GPS). You can buy receivers for these systems but they are much more expensive and/or currently impractical for our bushwalking use. For various reasons this will probably remain the case for the next couple of years. +The current practical accuracy of GPS for bushwalking under good conditions is about 100 metres horizontally,​ and much worse vertically. There are more accurate alternatives such as differential UPS (of which there are two basic kinds) and GLONASS (the Russian system which is roughly similar to GPS). You can buy receivers for these systems but they are much more expensive and/or currently impractical for our bushwalking use. For various reasons this will probably remain the case for the next couple of years. 
-To give you position, the GPS needs to gather information from several satellites (at least three to give a position without height + 
-information). Better accuracy is obtained if more satellites are used (under ideal conditions more than 12 are possible, but 5 or 6 are probably more typical in good locations). Most of the cheaper GPS units process the signals one satellite at a time. Some units are capable of processing several signals simultaneously. The advantage of this approach is that it enables the unit to give good results on poorer signals. For the bushwalker, this means that these units are much more likely to be useable in forests, valleys, etc.+To give you position, the GPS needs to gather information from several satellites (at least three to give a position without height information). Better accuracy is obtained if more satellites are used (under ideal conditions more than 12 are possible, but 5 or 6 are probably more typical in good locations). Most of the cheaper GPS units process the signals one satellite at a time. Some units are capable of processing several signals simultaneously. The advantage of this approach is that it enables the unit to give good results on poorer signals. For the bushwalker, this means that these units are much more likely to be useable in forests, valleys, etc. 
 GPS units get data from the satellites at a low data rate (50 bits per second). It normally takes 12.5 minutes for a satellite to transmit its whole message. To avoid having to wait this time, GPS units store information even when they are turned off. This way, it can take as little as 15 seconds or so for a GPS unit to give you a reading when you turn it on. It will take longer, perhaps several minutes, if the information it contains is not sufficiently accurate or is stale. GPS units get data from the satellites at a low data rate (50 bits per second). It normally takes 12.5 minutes for a satellite to transmit its whole message. To avoid having to wait this time, GPS units store information even when they are turned off. This way, it can take as little as 15 seconds or so for a GPS unit to give you a reading when you turn it on. It will take longer, perhaps several minutes, if the information it contains is not sufficiently accurate or is stale.
 +
 You can leave the GPS unit switched on and get a continuous reading of your position as you walk along. However, a new set of batteries will be drained in less than 10 hours (perhaps as little as 2 hours by some GPS units when used this way. You can leave the GPS unit switched on and get a continuous reading of your position as you walk along. However, a new set of batteries will be drained in less than 10 hours (perhaps as little as 2 hours by some GPS units when used this way.
-The GPS unit keeps very accurate time, which can be used by the UPS receiver to give you a very accurate clock. However this information may not be accurately displayed while the GPS unit is performing position calculations.+ 
 +The GPS unit keeps very accurate time, which can be used by the UPS receiver to give you a very accurate clock. Howeverthis information may not be accurately displayed while the GPS unit is performing position calculations.
 The price of GPS units is becoming more attractive. It is claimed that it is currently possible to buy a small The price of GPS units is becoming more attractive. It is claimed that it is currently possible to buy a small
 hand held unit (250 g with batteries) for $250 Australian. hand held unit (250 g with batteries) for $250 Australian.
 +
 The usefulness of GPS units in Australian bushwalking is not well established. Anecdotal evidence ranges from somewhat useful to almost useless. Considering the relatively low price of some units, it might be worthwhile for the Club to consider purchasing a unit for our navigators to evaluate. The usefulness of GPS units in Australian bushwalking is not well established. Anecdotal evidence ranges from somewhat useful to almost useless. Considering the relatively low price of some units, it might be worthwhile for the Club to consider purchasing a unit for our navigators to evaluate.
 +
 If you want to use a GPS unit as a navigational aid when bushwalking then you will want to use the unit in conjunction with maps. To avoid some pitfalls when purchasing or setting up a GPS unit, you should be aware of one or two basic facts: If you want to use a GPS unit as a navigational aid when bushwalking then you will want to use the unit in conjunction with maps. To avoid some pitfalls when purchasing or setting up a GPS unit, you should be aware of one or two basic facts:
-1. The earth is not spherical. The mapping authorities model the earth with an object called a spheroid. Different spheroids are used for different regions of the earth. The spheroid, in combination with other information gives a datum which is used when making maps. consequence is that the latitude and longitude of a given place depends upon which datum is used. In Australia the maps we use for bushwalking are based on the Australian Geodetic Datum 1996 (AGD66) or the later version (AGD84). + 
-2. With the datum, you get to latitude and longitude. For bushwalking we seldom bother with latitude and longitude. Instead we use the more useful map grid (which is based on the datum).A GPS unit which does not give your position in map grid coordinates will be of limited use to you. +1. The earth is not spherical. The mapping authorities model the earth with an object called a spheroid. Different spheroids are used for different regions of the earth. The spheroid, in combination with other information gives a datum which is used when making maps. consequence is that the latitude and longitude of a given place depends upon which datum is used. In Australia the maps we use for bushwalking are based on the Australian Geodetic Datum 1996 (AGD66) or the later version (AGD84). 
-If you purchase a GPS unit you should make sure that it supports either A0D66 or AGD84 datum (the difference between these wont matter to you, look for the words Australian, datum and 1966 or 1984). If your unit doesn'​t have these then you could use the standard WGS80 datum which is used internationally by the GPS systems, but your position readings (latitude, longitude and grid -coordinates) will have an additional + 
--abOirt,"​20:​6in'​etresIn +2. With the datum, you get to latitude and longitude. For bushwalking we seldom bother with latitude and longitude. Instead we use the more useful map grid (which is based on the datum). ​ A GPS unit which does not give your position in map grid coordinates will be of limited use to you. 
-liiiih ​to the correct datum, you + 
-will Want the unit to be able to give+If you purchase a GPS unit you should make sure that it supports either A0D66 or AGD84 datum (the difference between these wont matter to you, look for the words Australian, datum and 1966 or 1984). If your unit doesn'​t have these then you could use the standard WGS80 datum which is used internationally by the GPS systems, but your position readings (latitude, longitude and grid -coordinates) will have an additional ​error of about 200 metres
-+In addition ​to the correct datum, you will want the unit to be able to give you Australian map grid coordinates but the manual for your GPS unit probably wont mention the Australian map grid, however it will hopefully mention UTM (Universal Transverse Mercator) which is the projection ​used for the Australian map grid. In summary, the magic words to look for are Australian datum for 1966 or 1984, and UTM... 
-you Australian map grid coordinates but the manual for your GPS unit probably wont mention the Australian map grid, however it will hopefully mention UTM (Universal Transverse Mercator) which is the projecti6ii ​used for the Australian map grid. In summary, the magic words to look for are Australian datum for 1966 or 1984, and UTM... + 
-Once you have purchased your GPS unit and set it up, you can probably safely forget all of this, at least for the next few years (howeversee below.). +Once you have purchased your GPS unit and set it up, you can probably safely forget all of this, at least for the next few years (howeversee below). 
-Australia is currently shifting to a new mapping datum called GDA94. On the good side, this is for our purposes identical to the WGS84 now will be able to give horizontal position with the new datum. On the bad side, the new datum gives positions which differ horizontally from the old by about 200 metres (a different datum is used for height). This means that when maps are produced using the new datum you will find that all the features on them will have shifted by this 200 metre amount: the target date for implementation is the year 2000, but in fact the new datum will be phase4 ​in as new maps are issued. There are interesting times ahead.+ 
 +Australia is currently shifting to a new mapping datum called GDA94. On the good side, this is for our purposes identical to the WGS84 now will be able to give horizontal position with the new datum. ​ On the bad side, the new datum gives positions which differ horizontally from the old by about 200 metres (a different datum is used for height). This means that when maps are produced using the new datum you will find that all the features on them will have shifted by this 200 metre amount.  The target date for implementation is the year 2000, but in fact the new datum will be phased ​in as new maps are issued. There are interesting times ahead. 
 There you have it. GPS Units are available, at prices which aren't outrageous. They are potentially useful as a bushwalking aid to navigation, but not as a substitute to navigate with a map and compass. Care needs to be taken before making a purchase, for example one unit with one of the best reputations for use in bushwalkng-type conditions (because it can process several signals simultaneously) lacks the UTM transformation and hence is difficult to use with maps. There you have it. GPS Units are available, at prices which aren't outrageous. They are potentially useful as a bushwalking aid to navigation, but not as a substitute to navigate with a map and compass. Care needs to be taken before making a purchase, for example one unit with one of the best reputations for use in bushwalkng-type conditions (because it can process several signals simultaneously) lacks the UTM transformation and hence is difficult to use with maps.
-On a related topic,: AUSLIGhas ​released its Ai4stralia ​Unfolded CDROM for $99.95. However it is reported to have "no useful + 
-elevation detail to speak of at finer scales ie contours, mountains, etc" and is therefor ​"no good for bushwalking"​. Even with the addition of 3 kg of computer it wont replace-your maps!+On a related topic, ​AUSLIG has released its Australia ​Unfolded CDROM for $99.95. However it is reported to have "no useful elevation detail to speak of at finer scales ie contours, mountains, etc" and is therefore ​"no good for bushwalking"​. Even with the addition of 3 kg of computer it wont replace your maps!
  
 From the June 1996 issue of the Melbourne Bushwalking Club newsletter. From the June 1996 issue of the Melbourne Bushwalking Club newsletter.
199609.txt · Last modified: 2016/06/17 02:29 by vievems