RTK test, Aerial pictures accuracy, and OSM Database Accuracy

Posted by StephaneP on 11 September 2017 in English (English)

RTK accuracy

Since 1 or 2 years, I'm testing some low-cost GNSS receivers with RAW output. The goal is to get a cm accuracy. One way is to store the raw data, then post-process it with the open-source software RTKLIB. I had various fails and success and I finally find a point to place my own reference station, my "base" : base

One test was to put the "rover" on my car go back to my home. RTKLIB gave me a solution with "FIX" for a big part of the record : global solutionorange is "float" and green is "fix" (best accuracy)

An interesting part is a new roundabout, too new to see it on any aerial picture : New roundabout

Ok, but what about the accuracy ? So, zoom in, zoom in, ..... zoom more !! individual location Each square is 1 cm. Yes, the accuracy is about 1.5 centimeters !!

RTKLIB gave me a very good accuracy, but is this real ? I can't answer for this individual point, but with the results I got on surveys points from the French national geographic institute (IGN), I think I can say that the accuracy should be at about 5 or 10 cm, as my base coordinates are not perfect.

RTK vs Aerial Imagery accuracy

We are in September 2017, and the IGN just published new aerial imagery, and they usually do a very very good job. Finally I can see the roundabout I draw on OpenStreetMap 1 year earlier.

Let's compare my recording and the pictures: aerial vs RTK

Hey ! Not bad !! Now I must train to drive with a centimeter accuracy :-)

OpenStreetMap database accuracy

I speak about gnss trace accuracy, aerial imagery accuracy, but what about OpenStreetMap accuracy ? I heard that with 7 decimals, the coordinates stored in the OpenStreetMap database get only a 10 cm accuracy. Don't you think that It'll be a problem sooner or later ?

Perhaps we should consider adding a 8th decimal ?

Comment from tyr_asd on 12 September 2017 at 07:27

Very impressive!

Btw: 7 decimal digits already correspond to about 1.1cm.

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Comment from SimonPoole on 12 September 2017 at 07:47

Just a further note on increasing the possible accuracy: the 7 decimal digits are a consequence of using 32bit signed integers for the coordinates, changing that would be fairly involved.

In any case effects due to movements of the continental plates are larger (see ) than the limits on accuracy due to our coordinate representation and if anything should be addressed first.

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Comment from StephaneP on 12 September 2017 at 08:29

@SimonPoole Yes, the plates are moving, but it's a good idea to have a good relative accuracy. If a plate was moving at 10 meters each years, I still wan't to know if an object is 1 meter distant from another.

But I've calculate some distances on and tyr_asd is right, with the about 1 accuracy, so there is no problem. I think we have some time before we need a millimeter accuracy :-)

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Comment from Rinaldum on 12 September 2017 at 15:00

Dis, j'aimerais pas te rencontrer a la sortie d'un rond point :-D

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Comment from StephaneP on 12 September 2017 at 15:21

Ah bon ? Pourquoi ?

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Comment from philippec on 12 September 2017 at 20:11

He would miss you just by 1 cm.

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Comment from philippec on 12 September 2017 at 21:08

Where is the button I must push ?

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Comment from zstadler on 23 September 2017 at 09:52

Which low-cost RTK GNSS receivers are you using?

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Comment from Geonick on 24 September 2017 at 14:31

I’ve also heard from low-cost RTK GNSS which use Precise Point Positioning (PPP). Using these type of GPS mice (e.g. from u-blox) you don’t need a rover/base station and they reach sub-meter accurracy – especially when post-processed with RTKLIB: See e.g. .

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