Datums and things

[matt_e]

Probably about to expose my ignorance here but what the 'eck!



Gear:

iPAQ H2210

Etrex Summit GPS

Memory Map Navigator

TomTom Navigator 2



A list of queries in case anyone feels like enlightening me...



1) should I have the datum on the GPS set as WGS84 when using TTN2 AND Memory Map (supplied OS maps)? (That's what I am currently doing).



2) Does the 'Position Format' on the GPS have any bearing on anything else other than the display on the GPS? Similarly the 'distance/spd' and 'elevation' settings?



3) In Memory Map under overlay settings, do I need to change the 'units' & 'position' according to which map I am using or the GPS settings or is this just about the units displayed on the screen and for waypoints etc?



My reason for asking these questions is:



In TTN2 the timing of the instructions seem to be a bit variable. Even in places where I get 7 or more satellites, sometimes I have almost completed my turn before it says 'turn right' etc. Other times I get an appropriate few seconds worth warning to effect the turn. Also, I have noticed that sometimes it thinks I am on a parallel road to the one I am actually on. These problems are however inconsistent.



In Memory Map Navigator, the position seems to be some way off where I am actually standing (maybe 50 to 100m). On some maps, when I compare the supplied OS map with an Aerial Photo, the Aerial Photo seems to place me accurately whereas the OS map soesn't. Here is an example of the problem.



GPS map position



Any light shed would me much appreciated.



Cheers,



Matt

[Dave]

Datums I would always recommend setting to WGS84. It's the most common map datum used. Sometimes you may want to select OSGB, but WGS84 is what most software will be looking for. TomTom can sometimes give you late warnings, it really depends on speed, the accuracy of the fix you have, eg 4 sats or more, pref 7-9 and also at what point TomTom mark the map as a direction change. A typical situation is motorways, most motorway slip roads are marked as a turn at the top of the slip road, so if you're driving along the motorway and looking to turn off at the junction, if it's a long sliproad, you will get notified much later than you anticipate.

[matt_e]

Thank you Dave. I shall leave my GPS on WGS84.



I think part of my confusion is that around my area, the OS50 map does not line up with either the aerial photo tile OR an AtoZ road atlas. If you put a 'mark' on an obvious landmark on either of the latter they are pretty much in the exact same spot. However the OS50 is 'out' to a similar degree as in the photo above.



Is this because I live in a hilly area? Or is there some error on the map? It only seems to be on certain parts of the map and not all. e.g. put a 'mark' in the middle of the millennium dome and all 3 maps line up perfectly.



Any thoughts?



Matt

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