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Hi, I'm new to OSM, as you might have seen from my other recent post. I'm really keen on mapping the city of Kuala Terengganu, Malaysia, as there are a surprisingly large number of roads that are missing. The only up to date and reliable imagery for that area seems to be Esri World (not the Clarity Beta version) Imagery and Digital Globe Standard Imagery. The others are either too old (>5 years old, airport is different) or too low res. I'm having trouble finding the correct offset to use with the Esri World Imagery. If I line up the main roads near the coast, then the roads west of it seem to be not lining up. If I line up the minor roads towards the south of the city, then the main roads and the buildings at the city centre don't line up exactly. What should I do? Could someone please take a look? Here is the location: https://www.openstreetmap.org/#map=14/5.3080/103.1432 Thanks for the help. |
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There are some GPX tracks available in the area and a superficial check would indicate that they line up quite good with the existing OSM data (turn on the GPS layer in iD if you are using iD to edit). Varying offsets in the couple of meters range are normal and now and then you will find an "imagery boundary" with different offsets going right through the area you are trying to map. It's a pain but can't be avoided. In your case I would use the most current imagery and adjust to the GPX tracks (you can easily improve and generate more of those naturally). 2
just want to add on this: the offsets in aerial imagery could change continuously (not a hard boundary) over an area. Mostly if there are mountains. Just always use the nearest gps traces and check the alignment.
(30 Apr '18, 21:51)
aseerel4c26 ♦
Thanks Simon for your advice. I aligned the aerial map to the GPS traces within the area, and the majority of OSM roads in the area seem to line up decently. The GPS traces actually lines up quite nicely with the aerial imagery (offset -0.45; 0.60) overall, however there are a number of existing OSM roads (some quite long) that seem misaligned, even though the GPS traces are aligned. Perhaps they were created a long time ago with different imagery sources, so I might try correcting them (hope I'm right). Luckily, all the OSM data in my hometown in Australia seem to be aligned perfectly to the GPS traces and the aerial map. That city in Malaysia is not a hilly area at all (coastline city), I'm guessing the misaligned roads were done a long time ago.
(01 May '18, 08:17)
aaronshenhao
Also take note that the usual practice to map structures - for example, the buildings - are mapped with respect to their bases (which touches the ground), not their roofs. Confusing, isn't it? Shameless plug for the Malaysian community forum: https://forum.openstreetmap.org/viewforum.php?id=60
(06 May '18, 05:06)
AkuAnakTimur
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