[OSM-newbies] GPS vs Tracing

James Ewen ve6srv at gmail.com
Tue Nov 30 22:30:02 GMT 2010


Don't fret the fine details. The commercial mappers don't.

My Navigator using both TeleAtlas and NavTeq databases will
occasionally drag me away from reality. The unit has a snap to roads
feature to keep the little picture of a car on the road. It will
occasionally jump off the road and into the ditch because the
information from the GPS is too far from where the road shows up in
the database. I always trust the GPS location more than the map
database.

Orthophotography also has a number of distortions due to physics. The
Yahoo imagery in my area is out a number of feet. The GPS traces are
used to draw the roads, then you drag the photo to match if you're
going to trace.

Our consumer grade GPS traces match up very nicely with the government
commercial quality GPS collected data for verification.

If you're in urban canyons, under heavy tree canopy, or in real
cayons, then the GPS traces might be a little wonky, but with a decent
sky view, current consumer grade GPS units put you right on the money.

Try Geocaching if you doubt the veracity of my claims.

Like Richard says, "Get out there and have fun while mapping".

James
VE6SRV

On 11/30/10, Richard Weait <richard at weait.com> wrote:
> On Tue, Nov 30, 2010 at 3:44 PM, Donald Campbell II
> <donaciano2000 at gmail.com> wrote:
>> Okay so now Potlatch 2 is showing up on the Edit page along with Bing
>> background so I've taken a look at my area.
>> Since everything here was made from GPS tracks, how do I know which is
>> correct, the GPS or the overhead images?  Perhaps I was driving on one
>> side
>> of the road and drew it off to the side by a good 10 feet or so, perhaps I
>> had low accuracy...  on the other hand perhaps the aerial photos are off
>> by
>> a couple feet one way or another.
>> How can I tell if I should scoot my tracks to match the overhead or leave
>> them alone and assume they're more correct?  Also if the overhead images
>> are
>> off a few feet how can I get that fixed?
>> Thanks,
>> -Don.
>
> Hi Don,
>
> These are common questions, and common challenges for mappers since
> the beginning of geography, not just the beginning of OpenStreetMap.
>
> Firstly, every measurement comes with errors.  Every source, GPS,
> aerial imagery, lidar, diving rod, they all have inherent errors.  Any
> of the post-processing steps of format conversion, aligning to
> benchmarks, copy-paste, projection, can introduce further errors.
> Even if we check two different sources that agree, they might be
> coming from a common, errant source.  We have to know and accept that.
>  Frustrating, but true.
>
> Secondly, aerial imagery can never be as current as your own survey
> today.  At best, the aerial imagery was taken "yesterday", but likely
> the image is even older.  So if you observe a difference between your
> survey and aerial sources, it isn't always obvious if the difference
> is due to a change since the imagery was acquired, or errors from one
> source or another.
>
> So what is a mapper to do?
>
> Firstly, relax.  This is fun stuff.  Sometimes we have to decide,
> "I've mapped enough today."  Worrying over every last detail, or every
> last centimetre of imagined inaccuracy will extract the fun from your
> life.
>
> Secondly, map what you know.  Mapping places you see every day or
> every week allows you to use all of the local context that you have at
> your disposal.
>
> Thirdly, I like to use different guidelines for adding new objects to
> OSM and for editing objects in OSM.  When adding new objects, I feel
> free to use the source I best trust.  Ideally this is a combination of
> my own survey notes and GPX track, and aerial imagery if it exists.
> When editing an existing object, I'm much more likely to trust the
> work of the previous editor unless I know I have more-current
> information.  Leaving source tags for future mappers is a nice way to
> help them.
>
> None of this helps when you have multiple, disagreeing sources with no
> clear way to judge which is better.  So we do the best we can.  Split
> the difference?  Generalize with a node, rather than a polygon?  Pick
> one and describe with a note?  All of these work to one extent or
> another.  And the experience of mapping what you know best can help as
> you expand your range further from home.
>
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