OziExplorer track not accurate on Openstreetmaps

Hi Roberto,

OziExplorer is a different mapping & navigation app to Exploroz Traveller. I do not believe it has any affiliation with Exploroz.

Macca.

My guess is that your friend's calibration of the scanned map is off.

I did it once out of interest and although mine worked out ok, it's easy to get wrong. Having said that I did it on a small scale map, so a 300-400m error would not have been apparent.

Hello - as you have correctly identified would appear to be a datum (or possibly projection) issue , but unclear at what point the error is being introduced. Suggest you continue with the map (calibration) file itself. Here is another copy of the Hema Fraser Is map (calibration) file that uses AGD 66 as datum. Try that and see what happens. I haven't tested any track file (kml format) output with map.me as don't have time at moment but see how you go. (NOTE: Map file is from a Hema 2010 release in Ozf2 format - I have other versions if this doesn't match yours)

If you provide a Fraser Is track name that you are trying to replicate as kml in maps.me I can have a crack on my computer and send it to you the resulting kml - may also help isolate issue (if my map file doesn't fix it).



Cheers
Greg

The 5th line of the ".map" file should be:
"Australian Geodetic 1966,WGS 84, 0.0000, 0.0000,WGS 84"

if the map is to be read with AMG66 datum in Ozi.
There is no datum given in a ".kml" file, so WGS84 is being presumed.
The difference between locations in AMG66 and WGS84 datums is normally listed as around 250m, but maybe that would be enough for the error you're seeing?

Cheers, John

Going back a few years now, but I had a similar problem making my own maps from 1:25000 paper maps. Datum correct but everything was off a few hundred meters. Corrected it by manually re-calibrating (from memory) 3 or 4 points across each map and everything aligned correctly. It was not hard to do or to find the steps to do so in the manual. It was easier because I was mobile and I calibrated from the exact locations.

I would suggest finding an easily identified point on the digital map you are using. Then get its Lat and Lon. Using Google Earth locate the same point and compare its Lat and Lon. That would confirm the accuracy of the digital map. I use up to 8 points when calibrating new digital maps. Good luck.

Thanks All,

Re-calibrating the map myself did the trick. Not looking forward to do the other 149 maps btw, so I'll do them when I know for certain I am going to head into an area I am going to be needing Ozi tracks for.

Thanks again all,
Roberto

Good to hear you got it sorted Roberto.

Few observations:

(1) Cartographic products are not necessarily spatially accurate due to scale and symbolisation issues. For example a road and adjacent railway will often be offset marginally on a map for readability. Ditto for buildings etc which would form a blob on a small scale map if not slightly offset. This generally shouldn't affect watercourses most rural roads, and contours of course.

The cartographic representation issue will explain why GPS location data sometimes doesn't precisely match digitised map data. Rule of thumb is - the smaller the map scale the bigger the cartographic inaccuracies. For example digitising from a 1:10,000 scale map will be more accurate than using a 1:100,000 scale map. Pretty obvious.

(2) AGD66 coordinates were moved about 200m (give or take depending on your location in Oz) to the NE when the new Australian geocentric datum GDA94 was adopted. GDA94 and WGS84 were for most practical purposes the same (~1m diff). So, if you overlayed digitised data from an old map (AGD66) it would appear slightly west and south of its geocentric location before transformation to the new datum.



(3) The latest iteration of our geocentric datum, GDA2020, differs from GDA94 by <2m, again in an approx NE direction. This caters for continental drift but like the differences between GDA94 and WGS84 is irrelevant for most practical purposes - ie GDA2020=GDA94=WGS84

(4) All digitisation of paper maps must take the map projection into account when transforming. The most common projection for larger scale (smaller area) maps is the UTM - Universal Transverse Mercator - projection.

You only need to be aware that projections and datums are critical when converting (transforming) maps into digits, Software should do the rest.