RTK correction of GPS coordinates


It’s raining today, so no data capture ::frowning:
Actually the time window for best photos is surprisingly short, just a few hours either side of noon.

I have been disappointed with the accuracy of the GPS coordinated gathered with my phone (Galaxy Note 8). It is too painful to drag each point back on to a background image, there are thousands to do. So it would be better to have them in the right place to start with. I am not in urban canyons, just urban area but quite hilly with a lot of vegetation.

The first idea I have had is to use SBAS (WAAS or EGNOS) which should do better and there is a trial service in my area (NZ). This needs a better GPS that a phone because it does not appear that phone gps software uses this extra facility. My Garmin 76c handheld does, and it also has provision for an external aerial so that may help a little. I would then have to synchronise and postprocess the images. This is a bit fiddly because I would still have to verify the locations. Can I do this in the web app? Maybe we need to write something in QGIS to a better experience.

The next option would be RTK gps. This is now possible with the Trimble Catalyst aerial that has a USB output that allows a direct connection to a phone. Would Mapillary consider supporting this? https://catalyst.trimble.com/ShopNow/
I know it all costs extra money, subscription etc, but if we are trying to build a street furniture database, How can it be accurate if the photos are not properly calibrated to the nearest metre or better?

Are there any other options to improve accuracy?




Limitation of GNSS reception by smartphones is the size of the antenna that is too small. I’m not sure that adding dual-frequency will really improve accuracy, nor differential corrections (RTK, WAAS, EGNOS, the later two not applying to NZ). It would be easier to improve accuracy using an external GPS receiver like your 76C or my Etrex 30 but it is not straightforward because one have to synchronize smartphone pictures with external GPS track.


But we do have an experimental WAAS equivalent trial for Australia and New Zealand finishing in Jan 2019. I tried turning on the setting in my Garmin and it was much better, the track is always on the road! I have a small external aerial with a magnet that goes on the car roof. I agree that processing is more complex. I have to upload photos and the GPS track separately and synchronise. It would be nice to weed out the traffic light delays and visually check before uploading. I will work on something more streamlined. The same complexity if you have more than one camera.
More critical seems to be the lighting and weather. Some of my sequences are very contrasty due to the sun angle and direction. Having a rear camera would be even more limiting outside noon plus or minus a couple of hours.


The SBAS trial has finished, but new funding has been provided to continue the signal until a permanent service is in place. It appears to be on satellite 121 and that one does not show up for me on my phone. I have now been advised that the Android app GPSTest needs to know about it in an upgrade. I wonder if my Garmin really uses it, or it is just a better aerial on the car roof.

I thought that since a smartphone has a GPS that it would report GPS times in the EXIF record. But it is not there on my jpegs. If it was, we would not have to synchronize the times because we would have atomic clock accuracy everywhere.


My new Garmin Glo advertises that it uses SBAS and the phone GPS logging app confirms it. So I no longer have to adjust the raw location readings except for the occasional spike. Since it is a bluetooth device I can place the receiver on the dashboard for a better view of the sky. Not quite as good as the car roof, but bluetooth does not go through metal. Maybe i need to buy a convertible?