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Imagery Offset

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Ina Humeniuc edited this page Jul 27, 2021 · 1 revision

Different Imagery providers help in georeferencing their imagery, but occasionally the images can be out of position. This is due to when a flat image is stretched over an area of Earth with many contours resulting in misalignment of data from its true position by more than many meters.

Example - Way-i'd - w477916989, w477917009 pasted image 0

Offset Imagery compared to Maxar in Accra (Ghana) - Existing data created referring to Bing

Correcting Imagery Offset

METHOD - 1 : Public GPS Traces

Below are the steps followed: The best references for adjusting imagery are GPS tracks that follow roads. Since OpenStreetMap users often upload their GPS tracks to the OSM database, we can download them and use them to align our imagery.

  • Click on the download button
  • Check the box next to “Raw GPS Data” - This will download an additional layer to JOSM containing GPS tracks. Depending on how many tracks have been uploaded by OSM users, you may see few tracks (or even no tracks or many tracks)
  • Ignoring the box that pops up, use your mouse to drag the imagery layer so that it aligns correctly with the GPS tracks. The GPS tracks should line up with the roads on the imagery as closely as possible. You will see the offset numbers in the box change.

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  • If you like, you can save these offset settings by entering a bookmark name and then clicking OK. You can then automatically apply the same settings later by going to Imagery ‣ Imagery offset and clicking on your bookmark.
  • If you do not want to save the offset, simply click OK without entering a bookmark name.

METHOD - 2 : Strava Heat Map

Strava is an American internet service for tracking human exercise which depends on GPS, in mobile phones or other GPS-enabled devices, to record supported activities which can be shared among user's followers or shared publicly. Below are the steps followed:

  • Sign-up for a free Strava account with login credentials and then retrieve the following values from your browser cookie key values using
    Link : chrome://settings/cookies/detail?site=strava.comhttp://strava.com/ https://www.strava.com/heatmap#7.00/-120.90000/38.36000/hot/alls CloudFront-Key-Pair-Id CloudFront-Policy CloudFront-Signature
  • Add the following TMS entry under Imagery Preferences in JOSM, replacing MYVALUE with what you got from your browser

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METHOD - 3 : No Public GPS Traces / Strava

Identify the object whose position you know on the imagery which is Latest or Updated and now you can move the imagery as you did previously so that the feature you know is centered at the correct position. Make sure to use the Imagery Offset Database plugin which allows users to save offset information in the database and share it with others.

Unable to decide the imagery offset yet?

The OpenStreetMap Contributors Map aka Who’s around me is an interactive online map which will show you all volunteers of the OpenStreetMap world on a map. Each mapper type is distinguished by the number of changesets that were created. Based on the segment/changeset history one can drop a message/comment to the distinct mapper for further enquiry. Alternatively, contact community through preferred mode of communication (Telegram, Talk list, Facebook page etc)

How to identify any existing offsets recorded by other mappers

HOW TO CHECK IF THERE ARE ANY EXISTING OFFSETS IN JOSM

  • Install Imagery offset plugin : Preferences -> Imagery_offsets -> ok
  • Use the option “Imagery” - “get imagery offset” option in Josm to check if there are any existing offsets recorded by other mappers
  • If there are no offsets recorded check for the existing data alignment with imagery and strava heat map( process of using strava heat map is explained further)

HOW TO CHECK IF EXISTING OFFSETS CAN BE USED IN JOSM

  • Check for the offset distance in lower left corner and the date when the offset is recorded
  • Check if the same offset distance is recorded in the database - http://offsets.textual.ru/map. In Case of discrepancy please contact your lead
  • After validating the offset distance in both josm and database check for the number in the upper right corner
  • If this is greater than 2 km, it implies that existing offset is no longer valid and we have to create a new imagery offset

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Adherence to this process of checking existing offsets before start of mapping will ensure data consistency across any location

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