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I'm brand new and have read alignment posts and questions, but still am a bit stuck in place and afraid to make any edits. Here is the view in question. It is out of alignment with every image overlay in the database. As you can see, especially if you zoom in any at all, the buildings and many of the roads and service roads on the left side of the yellow secondary road are terribly out of alignment where the ones on the right aren't so bad. When I use the "align" tool, it sort of fixes the left side buy screws up the right and moves the main road out of the middle.

What is the best practice here, or should I leave it alone and let the experts deal with it?

I'm thinking I should:

  1. use "align" to make the major roads match the bing imagery and watch that it doesn't throw the overall alignment off terribly.
  2. fix the secondary roads to match the Bing image
  3. fix the polygons on the buildings.

asked 17 Mar '18, 00:54

mustang_guy's gravatar image

accept rate: 0%


Before moving anything it at all, it would be best to collect as many GPX traces as possible and decide then. It seems that there are no traces uploaded to OSM for this are, and the Strava heatmap is no longer working.

(17 Mar '18, 06:43) escada

Although there aren't any public traces in that immediate area (I can see one private north-south trace), if you zoom out you can see some. Looking at the edit history, some of the buildings you refer to have apparently been aligned to some (presumably older) Bing imagery.

Using the traces that there are to the north, east, south and west you may be able to work out which of the imagery sources is more reliable here.

(17 Mar '18, 09:58) SomeoneElse ♦

To check the alignment, or accuracy, of a area it is best done when there are lots of public traces. I would encourage everyone to upload their traces as public. One GPX may be a bit off but with several available a good average can choosen. Averaged way points can also be used. ( some GPSes can record a fixed position for several minutes improving the accuracy over time.) This Jpeg shows a roundabout nearby with all traces displayed (in CYAN). The Aerial can then be dragged into alignment, then the roads, tracks and other data. It is of course a problem if lots of mapping has been added using misaligned aerial backgrounds. So please upload those GPXes.

alt text

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answered 18 Mar '18, 11:54

andy%20mackey's gravatar image

andy mackey
accept rate: 4%

edited 19 Mar '18, 04:32

Andy%20Allan's gravatar image

Andy Allan

Both are important, realignment of a background layer as reference has to be done to seriously plot or correct misaligned elements in OSM.

As recommended, the best way to make sure there is no offset on a layer is to overlap several straight GPS traces meeting on a crossway and at least going to the four cardinal/intercardinal directions (Strava used to be a good reference) and align according the average. Beware that even GPS traces have an offset on curves, steep hills, near cliffs and because of the GPS tech precision limits (use Glonass in addition for a better result).

Also very important, beware of tilted satellite photo shots and steep oblique surfaces (steep hills, mountains, etc). The exact position gets deformed depending on the angle between the satellite photos and/or those surfaces. How to check:
- if there are steep hills, check on a topographic map.
- if the photo shots are tilted: search a close area with a building (higher buildings are better), the more you see its walls the more it's tilted, if you only see the roof, that's perfect. Select a less tilted image sat. source or realign the layer accordingly with the previous recommendation. Of course depending of the area the tilting changes on a same satellite imagery because of the satellite/Earth movements.

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answered 02 May '18, 10:59

SHARCRASH's gravatar image

accept rate: 0%

edited 02 May '18, 11:01

The wiki,, is horribly out of date. I have an iPhone SE. Is there a way I can use it to do the traces and upload them?

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answered 19 Mar '18, 21:14

mustang_guy's gravatar image

accept rate: 0%

edited 19 Mar '18, 21:36

SomeoneElse's gravatar image

SomeoneElse ♦


mustang_guy You should ask this as a separate question. I don't have a iPhone SE but i guess there is a way to upload your GPXes.

(19 Mar '18, 21:38) andy mackey

Software development never stops, so any documentation will always be playing catch-up. It does look as though that corner of the wiki could do with updating, either with information from or to just point at the information here.

Minh Nguyen (see the edit history of that page) is one of the people who've been trying to curate that page to keep it sensible; he may be able to recommend a selection for the bit of the beginners' guide that you're suggesting updating.

(19 Mar '18, 21:41) SomeoneElse ♦
(19 Mar '18, 22:08) andy mackey

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question asked: 17 Mar '18, 00:54

question was seen: 3,382 times

last updated: 02 May '18, 11:01

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