Imagery Wins Over GPS?

Posted by Dion Dock on 19 August 2010 in English (English)

I am developing mixed feelings about how to map for OSM.

I've made traces of roads that don't appear in Yahoo! imagery with my GPS and uploaded them. However, the mailing lists have provided WMS imagery that can be used in JOSM to achieve the same result. Heck, it's usually better than the GPS track. It is also much less work to trace an area in JOSM than it is to go out there, traverse the roads, upload the track and trace an image from it.

There's a feel good component to "beating" Google/Bing/Yahoo with a new road. But it starts to feel like wasted effort when I can find imagery that is new enough.

Worse, manual effort seems like a waste when bulk data can be uploaded. For example, if Portland OR has data on which roads have bike lanes, it could probably be part of a bulk upload. Meanwhile, I'm pinging away tagging bike lanes as I find them. Ditto for hiking trails: why bother to add them by hand when someone might do a bulk upload from US Forest Service data?

Does anyone else have this feeling? Imagery rules over GPS? Freeing data for bulk upload or adding little bits by hand?

Comment from wallclimber21 on 19 August 2010 at 01:04

In case there are aerial pictures and the street topology matches the TIGER data, then GPS traces are not all that useful. But for cities that saw a lot of expansion during the housing boom, the TIGER

Comment from wallclimber21 on 19 August 2010 at 01:05

... the TIGER data will not be up to date. That's the time to get out the bike/car and start filling in real names, in which case you might as well record it with GPS too.

In case of hiking trails: I've seen the bulk loading case when hunderds of self mapped trails were replaced by a bulk load from the local park authority.

Well... it sucks a little bit, but the end result is better. And that's what it's all about, isn't it?

Comment from andrewpmk on 19 August 2010 at 04:41

If the imagery is properly licensed, by all means trace from it. Of course, surveying is very much encouraged since the imagery may be out of date/unclear and some things cannot be added from imagery (road names, small businesses, house numbers...)

Comment from skystis on 19 August 2010 at 06:01

Best solution is GPS traces + pictures + wms. Using only wms you can't know where is gate, shop or some patch covered by trees.

Comment from smudge on 19 August 2010 at 08:47

I guess fairly logically, I personally find WMS imagery useful for outlining the major features. In particular some things like wooded and land use areas which are not necessarily easy to walk round with a GPS. The GPS adds details that can't be garnered from imagery (and I would argue its these details that make apps and tools that are based on OSM data so much more useful day to day) and it also in many cases is much more up to date than much of the imagery.

Comment from andygates on 19 August 2010 at 09:09

Imagery never catches ground-level detail like turn and vehicle restrictions, so boots (or tyres) on the ground are always the best. And even the shiniest, spankiest WMS layers (in the UK that's the Ordnance Survey streetview) has its flaws: I've come across incorrect street names and recent developments in the past week, local to where I was holidaying.

Comment from dcp on 19 August 2010 at 17:43

The nice thing about OSM is that each mapper can do:
a. What he wants to do (like hiking ways, motorways, bridleways, MTB routes, etc.)
b. in the manner he/she is best at.
I personaly am a hiker so I specialize in tracks and paths with the associated POIs. Now a lot of forest areas have been entered by other mappers probably using aerial imagery and believe me it is often very wrong. Landuse boundaries, POIs, streams, ditches, etc. are navigational aids to hikers (all who have used Ordinance Survey Maps know that) and this information can only be gathered in the field itself.
Our GPS-gear is only accurate to +/- 3 meters but you can overcome that if you download all the GPS-Tracks to JOSM and then interpolate the trend.
Many main roads where I do my mapping have abrupt turns (i.e. not smooth bends).
This could also be a result of aerial imagery.
Think about it like this: Surveyers are in the field gathering data. The get paid for it; we do it for fun. (The second golden rule of OSM: HAVE FUN!)

Comment from PJ Houser on 23 December 2010 at 01:00

"For example, if Portland OR has data on which roads have bike lanes, it could probably be part of a bulk upload. Meanwhile, I'm pinging away tagging bike lanes as I find them."

So this is exactly what Trimet would like to do. I'm on the newly hired team at Trimet to get relevant routing (bike routes, centerlines, sidewalks, trails) public domain data from RLIS, Portland, and other agencies (see for available datasets) on to OSM. Trimet is switching to open source trip planner ( and open source data (OSM). If you or anyone has experience with bulk imports or simply has input, please email me at I would love some input!

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