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I am just back from a holiday in Morocco, I have recorded tracks of my holiday using a N900 and at least in Tamegroute, I also took some field notes about smaller details that could not be revealed by the GPS.

now I am comparing my tracks with the background imagery displayed with potlatch-2 and and JOSM (on potlatch-1 I don't know how to set it) and I notice a systematic offset.

  • what is the correct action to take in this case?
  • is my GPS to be trusted or the alignment of the imagery?
  • in Marrakech and other areas, I do not have this problem (but pity I did not take field notes!).

asked 18 Jul '11, 10:46

mariotomo's gravatar image

accept rate: 0%


Have you got high res bing imagery where you live? Try leaving the GPS on for a bit, and then go for a walk around your local area - see what the trace looks like...

Also, were the tracks made in a car, or on foot? Was it in open sky, or were you in a forested or urban area with tell buildings? How often was your track sample rate?

I've found occasionally that a trace can be a way misaligned for no real reason. Similarly, aerial photos can be misaligned...

Try seeing if there are any other GPS traces in the surrounding area.

(18 Jul '11, 13:16) c2r

this is the permalink to the area: the only tracks there are from my N900.

tracks made both in car and on foot, mostly open sky, definitely no tall buildings, some brief parts in covered passages.

all tracks have the same average offset as compared to the background image.

(18 Jul '11, 14:11) mariotomo

Thanks for the link - it makes it much easier to see what's going on. Agreed with andy's assessment below..

(19 Jul '11, 08:00) c2r

If the offset is always the same over the course of a few days, it's a good indicator that the error is in the aerial imagery, not the GPS device. So you can apply an offset to the imagery in your editor, and continue editing from there. In potlach2, as andy said, hold the space bar to align the imagery. In JOSM, right-click on the bing layer to set (and bookmark) an imegary offset.

permanent link

answered 18 Jul '11, 14:23

Vincent%20de%20Phily's gravatar image

Vincent de P... ♦
accept rate: 19%

will try potlatch2 from my netbook. from my office laptop I only have potlatch1. I am not confident yet with JOSM.

(18 Jul '11, 15:24) mariotomo

... and in Potlatch 1 (as in Potlatch 2) holding the space bar allows you to drag the imagery around underneath.

(18 Jul '11, 15:42) SomeoneElse ♦

In Potlach2 holding the space bar allows you to drag things into alignment. This is a case where lots of traces give a good average for alignment you can only do your best with what you've got. A systematic off set is good as it means dragging will put thing right. as the comment say if you walk around the block a few times or keep the gps still for a while outside the consistency of the trace will give an indication of it quality

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answered 18 Jul '11, 13:17

andy%20mackey's gravatar image

andy mackey
accept rate: 4%

edited 18 Jul '11, 13:28


In JOSM under imagery there's an offset adjustment if required

(18 Jul '11, 13:34) andy mackey

I have looked at it with P2 and I think you have a solid case for moving the image to match your trace then you will have to drags all the streets and other data to match the new alignment.maybe wait for second opinion for a day or so in case some as encountered a mass offset of a trace and can explain it. I am not sure if JOSM or P2 is best tool for job,but probably JOSM

(18 Jul '11, 15:30) andy mackey

Its not a lot of data to move but some screen prints (of image and present points from open editor screen) and notes could be useful,allow your self one and a half hours. I think Potlach 2 will be fine good luck

(18 Jul '11, 15:57) andy mackey

Either your GPS or the imagery could be wrong. Without strong evidence either way, leave the data as it is.

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answered 19 Jul '11, 05:08

Stevage's gravatar image

accept rate: 0%

Stevage, mariotomo's trace shows that he spent some time in the town with several traces along streets with the indications that he spent stationary time, which in effect averages his position which to me confirms the traces are good.the only thing I would add it is maybe he could send the original mapper a call and tell him why and what youv'e done.

(19 Jul '11, 12:01) andy mackey

also: in my own editing session I have offset the background imagery, but in fact I behave as if I had a strong argument for stating that the background needs better alignment. how do I tell the community?

(19 Jul '11, 14:15) mariotomo

I've just been reading this hope it helps

(19 Jul '11, 18:00) andy mackey

If there are multiple traces on different days confirming the offset, that's definitely "strong evidence". In that case, moving the roads seems justified. One weakness in our system is that there's no good way to record that offset (the "True Offset Process" is still vapourware), or why the roads moved, to prevent future OSMers undoing it.

(20 Jul '11, 03:36) Stevage

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question asked: 18 Jul '11, 10:46

question was seen: 8,793 times

last updated: 20 Jul '11, 03:36

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum