GPS Position Formats What do they mean?

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1 GPS Position Formats What do they mean? in the Degrees Minutes and Seconds speaking world universally means 23 Degrees 9 Minutes 54 Seconds but what is it a measure of? Maybe a bearing, horizontal angle, vertical angle or maybe a line of latitude maybe even longitude? Without some form of identification the type intended is difficult to correctly determine. Now , immediately suggests a position of latitude/longitude but in which hemisphere and is the longitude East or West? With local knowledge and knowing the approximate local coordinates may solve the intention but in the global world of today should we have to make assumptions? So S, E clarifies the intentions and eliminates any ambiguous assumptions that one could make from the above. Apart from the lack of Datum issue, which is another subject and not covered here this Position Format gives a clear indication of the position in a global context. However different variations of the Degrees theme are used world wide depending on the purpose. Marine type formats tend to use Degrees Minutes and decimal minutes (ddd MM.m ), which may be a flow on from the de-facto NMEA (National Marine Electronic Association) standard commonly used worldwide for data transfer between electronics equipment. Bureau of Meteorology s tend to report meteorological conditions in Degrees and decimal degrees (ddd.d ) while other organizations (AusSAR etc) also use Degrees and decimal degrees (to one decimal place) for search and rescues. Then there are charts and notices to mariners that use both ddd MM SS and ddd MM.mm type formats. Add to that the maze of sometimes confusing formats that can be found in some fishing magazines and websites it is no wonder that there could be confusion. So what does all this mean for the occasional GPS user in a recreational type environment? Confusion, well sometimes Yes, but particularly when the written or spoken format is open to interpretation or misinterpreted by users for a number of reasons. In emergency situations the many different organizations involved may not be all talking the same format language. In some instances several kilometres even 500m is not important but in other cases just being in the right street might be critical. With the setting of Selective Availability to zero (May 2, 2000) position formats are now of more importance considering the current accuracy available from the GPS system. Kerry Matthews Page 1 7/01/2015

2 There should be no confusion or assumptions made of the intent at any level but unfortunately this is not always the case. Take the position S, E and the possible relayed variants As intended (well sort of). Note decimal point Minus signs? But is it D M S or D M.m? Decimal points? Is this D.d, D M S, D M.m? One would expect this to be D M S but is it? Decimal degrees? but could be anything! And what about the following, which appear close but how to be sure! S, E S, E 54 & 33 Decimal minutes or seconds? Safe to assume what? Not entirely sure Obviously the intention can be unclear with both the written and probably even more so with the spoken format. Professionals have standards and methods, which eliminate a lot of this confusion but for the occasional GPS user (hiker, fisherman etc) this may not be the case. These example types were all based on position formats from recreational magazines and web sites so they are real world formats. For instance take the example ( , ), which has an obvious Degrees Minutes Seconds format about it but off course the intention was (we are assuming) Degrees Minutes AND decimal Minutes. This may be possible depending on the method relayed and the thinking of the other party. The GPS settings format must be configured correctly to accept the intended / actual latitude & longitude. Several years ago this situation was highlighted as I listened to GPS positions being relayed over the VHF radio. On it went all were spoken specifically as Degrees Minutes & Seconds and all sounded well until the magic one of, some many Degrees so many minutes and 99 Seconds. Suddenly it was very evident that all the previous positions were actually Degrees Minutes and Decimal minutes but relayed as Degrees Minutes & Seconds. So what could that little oversight mean? Kerry Matthews Page 2 7/01/2015

3 Figure 1 Affect of Assuming Formats Figure 1 highlights the difference in this example with mistaking the position format D M.m for D M S or maybe inputting the values with the incorrect GPS format configuration. Note the actual correct formats and position relative to misinterpreting , as either being D M(.)m or D M S and all because of omitting a single decimal (.) point. The error in position will also vary in size and relative position depending on the actual latitude/longitude values. In this example the error in position is approximately 800m (half a mile) and even on the wrong side of the headland! One would expect that if the values were equal to or greater than 60 (as 60 seconds = 1 minute) then the assumption would be that the format indeed was D M.mm. However in some cases many available fishing marks are predominately under 60 seconds (or <0.60 ) and only displayed to either 2 decimal minutes or what could be mistaken for whole seconds. Perhaps this is coincidence but then perhaps they may have all been DMS format in the first instance, and now displayed in suspected D M(.)m format? This is probably something one will never know. Apart from the fact that this Kerry Matthews Page 3 7/01/2015

4 type of occurrence is setting a bad example in educating the recreational user a lot of these marks should be treated with suspicion. A case of use at your own risk but then it really takes no more effort to do it right in the first place to clarify exactly what one is intending. Add to this the effect of different operating Datum s plus factors such as rounding and the error in position could increase substantially. So what is the effect of rounding? Rounding affects the capability of how close one will effectively (all things being equal) get to the intended position. Imagine you want to measure a length of metres. Ok with a ruler marked only in whole metres then the closest whole metre would be 2 metres. Now with a ruler in metres and decimetres (1/10 of a metre) one could accurately measure 1.6 metres (the nearest decimetre with confidence). Divide the decimetres into centimetres and one could make it either 1.58 or 1.59 metres And for those who prefer an imperial example 1 foot 6 and 11/16 inches. With a ruler marked only in whole feet then the closest whole foot would be 2 feet. Now with a ruler in feet and inches one could accurately measure 1 foot 7 inches (the nearest inch with confidence). Divide the inches into 1/8 of an inch and one could make it either 1 foot 6 and 5/8 inches or 1 foot 6 and 6/8 inches So as we divide the measurement capability into smaller components we can achieve a more precise result dependent of course on the accuracy. Similarly this principle of breaking down the measurement into smaller and smaller measurements (increasing the decimal places and potential measuring precision) applies to latitude / longitude positions. Figure 2 shows the required number of decimal figures for both D M.mmm and D M S.ss formats that are required to achieve certain metre accuracy limits. This clearly shows that even within different ranges the accuracy capability can vary and is not constant. For instance the position error using D M.m format to 1 decimal minute can vary from about 26 metres to well over 250 metres. Undoubtedly this format to 1 decimal place wouldn t be suitable to reliably Kerry Matthews Page 4 7/01/2015

5 locate a GPS (and especially DGPS) position if one expected to achieve the systems and the equipment capabilities especially with SA terminated. Figure 2 For a Degrees Minutes and decimal minute type format (D M.m) the minimum acceptable would be at least 3 decimal places for standard GPS & DGPS type positions. Decimal Degree formats (D.d) also require some consideration especially when converting between different position formats. Precision to five (5) decimal places should be considered the minimum. Now that Selective Availability has been set to zero and especially with regards the future accuracy potential of the GPS system manufacturers might have to implement increased position format precision. It would be nice to standardize Position Formats to meet accuracy requirements while eliminating confusion that could exist between the many different users of geographical data especially with emergency services and GPS navigation. This could be a goal for the future where all users are on the same wavelength but in the meantime users should be aware of formats, accuracy capabilities and the effect that misleading positions may have on their own and others safety. Kerry Matthews Page 5 7/01/2015

6 K.J Matthews - 3/08/2001 (Original version 25/3/00 revised for SA) Copyright 2001 by Kerry Matthews (All rights reserved) This report is the sole comments and opinions of the author and may be inconclusive without further analysis and/or investigation of the data. Copyright and shall not be altered or reproduced by any means - mail to mattk@tpg.com.au Kerry Matthews Page 6 7/01/2015