Submitted: Sunday, May 01, 2005 at 21:44
ThreadID: 22558 Views:2189 Replies:4 FollowUps:2
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Hi All.
Just got back from a trip out bush.When useing Oziexplorer and Natmap in moving
maps found my pointer just off the track, But when useing Hemma desert tracks
the pointer was on the track. Would this be because of the map scale?
Also saved my track on the Laptop, when I bring this up on Natmap it is where it
should be. but when I bring it up on hemma map it is way out (about 70km)
Any ideas what could be out of wack. cheers Brian
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Reply By: geocacher (djcache) - Sunday, May 01, 2005 at 23:04

Sunday, May 01, 2005 at 23:04
Make sure your datums in the map header box are correct for the map you are using. Check that the datum in config in Ozi matches that the GPS is outputing.

The differences between two maps when in use usually relate to scale and size. The 250k seamless image calibration is a compromise across hundreds of map sheets and will be less accurate than even the single sheet of the same map series. The bigger the area the map covers the less accurate it will be at a single point compared to a map of a finer scale. (eg. Compare a 1:250k map and 1:25k map of the same area.) This is due largely to the fact that the earth surface is curved but you are trying to display and calibrate it as a flat map. The smaller the area you calibrate the less of a factor this is.

The most important thing of all though is how many waypoints are displayed on your screen indicating how many geocaches you have to choose from in the area you are visiting -



AnswerID: 109152

Follow Up By: Utemad - Monday, May 02, 2005 at 09:46

Monday, May 02, 2005 at 09:46
If you had calibrated the maps yourself I would have thought it was a datum issue. However considering you would have used the files from the Ozi website or the ones on the CDs I wouldn't think so. If you did download the map files are you sure they are the right ones?

Does sound like datum though.
FollowupID: 365883

Follow Up By: Mad Dog (Australia) - Monday, May 02, 2005 at 22:16

Monday, May 02, 2005 at 22:16
>Check that the datum in config in Ozi matches that the GPS is outputing.

Yep, this caught me out when I first started out with this stuff. Now the only time I'm off track is after fiddling around and I know the first thing to check.
FollowupID: 365975

Reply By: Member - Jeff M (WA) - Monday, May 02, 2005 at 13:04

Monday, May 02, 2005 at 13:04
Mines always on track, however I use a 12 sat Garmin.
AnswerID: 109219

Reply By: TomZ - Monday, May 02, 2005 at 19:28

Monday, May 02, 2005 at 19:28
NATMAP maps are cartographic based products which means roads and other topographic features on the map are displaced from their ground truth positions and generalised according to the scale of the map to make the map readable. Also in many cases new information is obtained from satellite imagery (SPOT 4&5) and put on a map with less accuracy than the original information which was put on by photogrammetric means. Features are never going to be as accurately positioned as a GPS will position you. This same issue exists with the NATMAP Raster products which were also derived from the same source as the new 1:250,000 (250K) scale maps. From memory the average horizontal accuracy of featrues on a NATMAP product at 1:250K is +/- 125 metres. Geoscience Australia is very careful to state the accuracy of their NATMAP products if you read the blurb that goes with their products.

I suspect the HEMA desert tracks were probably positioned on their maps using GPS tracking. I know HEMA have at times provide their GPS information to NATMAP for use in NATMAP products.
AnswerID: 109277

Reply By: pjchris - Monday, May 02, 2005 at 21:58

Monday, May 02, 2005 at 21:58
In the new NatMap 2005 Premium edition, +/- 125 m equates to +/- 3 pixels as on the digitised maps 1 pixel = 29m. On the older NatMap 2003 1pixel = 32.5 m. So if you are within that range of the tracks/roads then you would be within the expected accuracy.

However if you are further out than that and it is consistent then there is a proble with the map calibration and or datum when imported in to Ozi.

Tom is correct in that map features such as roads, tracke etc can be displaced but this is only done when they coexist on the map with a higher priority feature. There is a list on the GeoScience Australia website that explains which features have priority and are always in their correct location and shows which ones will be moved. Buildings are at the bottom (or least important) end ofthe list and watercourses are the highest priority (I Think).

Did you import the maps direct? Or did you download the Map calibration files from the Australian OziExplorer website? I believe the ones on the Ozi website are more accurate and set the map corners to the map area so as to exclude the borders of the mapsheets.


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