Highest accuracy in Africa - no cors networks

Hi everyone. I’d just like to ask some advice and tips here regarding using the Reach RS to improve gps accuracy for aerial surveys using drones. I have a system that integrates well with the Reach unit (tuffwing mapper). I have purchased both a Reach RS and the Reach unit to mount onto the drone.

I am based in deep dark Africa, I will be conducting a survey in the middle of Mozambique, the nearest town is a few hours drive away. I would like to know the best possible way to achieve the highest possible accuracy. For this survey it is a general topographical survey and accuracy doesn’t need to be absolute, but obviously I have the chance to test it out and would like to try achieve the highest possible accuracy.

I am able to mount the Reach RS unit in a secure location and leave it for a day or possibly longer before I come to conduct the survey.

I have also read that if I know the absolute position of the Reach RS base - I can use this is adjust the position it is detecting. The Reach RS will then accurately know exactly where it is to absolute accuracy, and then the rover unit will have relative accuracy based on this location.

I have tried to look into online CORS networks. But due to the remote nature of where I operate, I haven’t been able to find any online sources to use.

If anyone has any tips or guidance, or potentially point me to a link or two that can explain specific to my situation that would be much appreciated.

Thanks

Howzit fellow African!

So I have done a fair bit of work in your part of the world. I think we need to clear up first by what you mean by accuracy. You can get relative accuracy and absolute accuracy. For relative accuracy, you will be limited to the accuracy of your GPS system. So it you measure 2 points on the ground with your RS and then with a tape measure, the distance will be the same. For this method you dont need any external control system to provide a position for your base. Just suck a single, uncorrected position for your base and away you go.

Absolute accuracy is a bit different in that the system is relative to an established coordinate system like UTM or similar.You survey should then be similar to the one that someone did next door since you are using the same coordinate system and lot a temporary, unfixed one. But if needed and you have to get an absolutly accurate survey, you are in a bit of a jam which is a limitation of a L1 only receiver. You are far to far away for any CORS to work properly. (> than 20km baseline I assume). There are a few control points published, but these are most likely destroyed, or on a local system. There was a private CORs called MozNET but I think its only for Maputo.
Setting up you receiver to collect data for a long period of time doesn’t help anything. You just collect an average of errors that are not corrected anyway. You will still be 2-3m from WGS84 any way.

If you dont need to provide an absolute accurate survey, then dont worry about it. At least use a proper Geoid model for your survey as you will be about 25m off your real height vertically. And place some proper marks on the ground that will last. You never know when you or someone else will want to revisit this survey of yours and will want some information about how you determined your system and how to "calibrate’ to it.

Good luck!

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If you need absolute accuracy on a coordinate system you could try Precise Point Positioning with NRCAN (https://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php). You will need to setup your base and let it collect plenty of raw data (many many hours) as you take survey points with the rover. It’s not as accurate as a good CORS network, and it can be painful waiting to collect enough base data, but it is better than a standalone GPS on single. I think you can get it down to around 0.5m accuracy with an L1 receiver alone.
But I agree with Luke regarding physically marking out points at site you can go back to. Some of the work we do is in really remote locations where established coordinate systems are thrown out the window. Physical reference points are key!

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