GCPs & GNCC Basic Questions

Hoping someone on here can give a fairly novice mapper some help with some details. I have flown some mapping missions, but these were mainly for people that wanted a more detailed view of their properties and great levels of accuracy were not needed

I am running into requests for greater accuracy and would like to meet the need. From what I understand, although your drone collects geolocation data, it does so with a fairly large tolerance in term of accuracy. Using GCPs helps with this, but only so long as the placement of those points have a greater level of accuracy, and in turn using a simply GPS unit (similar to what is on-board the drone) is not really providing any additional accuracy value beyond there being a second set of reference points, as these points are only accuracy to 10 meters as well.

In turn, if you are looking for greater accuracy you can step up to a handheld unit like Bad Elf GNSS Surveyor will get you within a meter, and higher end units, like their Flex or Ublox ZED-FP9 will get you sub-centimeter. Clearly, I am looking at budget solutions…I know there are some better and more costly units out there.

Correct so far?

My big questions are:

  • Is the value of using a base and rover setup so that you can place the base at a known, absolute positioned point like a surveyors marker, and use that as a point of reference for the rover unit to connect to, and in turn get an even greater level of accuracy for setting GCPs with the rover?

  • Is the base unit required, or could you just use a the GNSS unit to grab coordinates of each GCP you lay down? If so, do you need to have a GCP at an absolute positioned location as a reference point to make adjustments of other GCPs?

  • I do a lot of work in fairly remote and hilly terrain. Quite often there is no cell phone reception, but can get GPS connection. What mechanism do units like ZED-FP9 use to communicate with one another, and what is the range between the base station and the rover? Isn’t terrain, line of site, and physical barriers going to impede connection?

You may know this but let’s get something straight up front. In terms of mapping you have relative (site) accuracy and absolute (global) accuracy. Absolute accuracy requires a known point tied to the real world or a reference collected from a CORS station via NTRIP. Relative accuracy is achieved in this case but absolute accuracy is not guaranteed by achieving relative. Geographic surveys always require absolute whereas grid or something like a construction site requires relative accuracy and will probably require further alignment to the project datum.

This is correct. You can relate the native GPS to your cell phone. At the very best it is reporting +/- 1m horizontal and 1.5m vertical. There are several ways to achieve varying levels of relative accuracy that is better than the native GPS.

1.Google Earth with the SPCE and GEOID12 plugins

You can get better vertical results on this by running a traditional level loop.

  1. Use the first map. This won’t help the overall map accuracy but if you set targets that are visible you can extract values from the first map and use those as GCP’s in subsequent maps. This allows relative measurements to be made and to keep everything horizontally aligned. You can do this in conjunction with the Google Earth method and get within about 50 centimeters.

  2. Use a mobile device app that does averaging. This one depends on how much patience you have because you need to place a mobile device on the point and let it average for about 5 minutes. This will get you within 0.5m relative accuracy and 1m absolute global accuracy.

Beyond that you really need GNSS equipment and that gets even deeper as you try to achieve even better accuracy.

Yes. A local base/rover setup will get you centimeter relative accuracy on the GCP points and RMSE’s of around 20cm.

You could use just a rover but you would need to occupy each point for about 30 minutes and then use a PPP service to nail down the absolute accuracy.

Most GNSS receivers communicate via LoRa (long range radio) which is just more powerful wifi. Line of sight can become an issue. Also you are susceptible to obstacles in the area where you might lose satellites. When this becomes a problem or you are running NTRIP and are in an area with no data connection you will learn to PPK. In the case of a local base you can acquire a local coordinate for your base from anywhere within a couple of kilometers of the subject area and traverse in a point to that area from further out. This is particularly helpful if you are using NTRIP and cannot get reception close to your site but can not too far away. Basically you get the point via NTRIP or entering the known point manually, setup your base at that location and then use the base/rover to set two points nearer the site so you can move you base to there. Two because you should always have a check. Then you can collect your points.

In my opinion the best kit for the money right now is the Emlid RS2. They are very reliable and maintain a strong connection whether you are on local radio or NTRIP to CORS. They are near half the cost of their main competitors and a tenth of the price of a major manufacturer like Topcon or Trimble. For what drone mappers need those are way overkill. Emlid also has a product that you can put on a standard Mavic 2 Pro or Phantom 4 Pro to do PPK. I hope this gets you pointed in the right direction.

Thank you very much. I’ve absorbed 90% of this. The other parts are still bouncing around my skull but I’ll dig and learn without using up more of your time. I’ll also take a look at Emilid RS2 system. UBlox looks interesting but also looks like a bit more of a science project than I have time for right now.

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Any time. Feel free to PM me and we can work through it without bashing your thread. The Emlid systems use UBlox better are ready to go out of the box. The kit for the drone has the Emlid M2 and antenna but needs a mounting kit. Personally I think learning to PPK this way is the route to go. You would then only need one elevation on site to calibrate you elevations to. Checkpoints should be used but I don’t know if they are absolutely necessary right now for what you are doing. Eventually you’ll want to end up with a PPK/RTK drone and leave all this behind.