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How to establish GCPs using PPK?

I’m planning on getting an Emlid Reach RS and utilize CORS to establish precise coordinates for ground control points used in photogrammetry (drone surveying). I’m using GCPs because I don’t have a remote sensor setup on my drone yet. I’m new to this and I can’t seem to locate a “How To” for how you go about taking the data from the Reach taken at each of the control points and processing it with the CORS data to correct the coordinates. I don’t even know how close the CORS station needs to be. I found the Reach documentation but it appears to explain how to use it (actually use a pair of them) for RTK not PPK. My understanding, and maybe I’m misunderstanding, is that to establish absolute coordinates using PPK I don’t need two units (just one unit and data from a nearby CORS station) However I must admit it baffles me how this can happen without having at least one initial point to place the Reach on that you know the absolute coordinates already. But I think that’s just due to me still learning about PPK.

Would greatly appreciate it if someone can provide a link or otherwise throw me a bone on this. Thanks!

Dear Jeff,

Regarding the use of the products you mention, we recommend you contact their support, or wait for the input of other users.

In the meantime, you can check out the post from one of our users, briefly going over the difference between RTK and PPK, using CORS and describing his experience: Although the post is pretty old, you can try adding a comment to the thread to engage other users.

Also, you might be interested in our article about acquiring ground control points. When importing them into Pix4D Desktop, make sure the accuracy values (Accuracy Horz and Accuracy Vert) you set reflect correctly the accuracy of your ground control points coordinates. 

To learn more about adding/importing ground control points, click here.

To learn more about setting the accuracy for ground control points, click here.


If you are going to use just one Reach RS unit you will need to log raw data at each point at least 1 hour to get the necessary logs for PPK. Nearest CORS station should be closer than 100km.

Personally I use 2 units. One as BASE and the other as ROVER.

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1 hour!?  What information do you base your 1 hour requirement on?  Aren’t there variables that affect the requirement for how long you need to log data such as the precision you are going for, etc?  

When you use two units, the location you place the base needs to be a point that has already been surveyed, no?  So if you have no survey points to place the base on, then seems having two units is useless, no?


Let’s get some facts straight first. Are you going to use your Reach unit for RTK, PPK or PPP? Do you need absolute or relative accuracy?

You can use local CORS station to get the correction data in real time (usually paid service) and you’ll need just one Reach RS unit to get the absolute accuracy (RTK). 

You can use 2 Reach units to get relative accuracy and palace the base using average single position. You can set the time to average the coordinates for the base 0,1 - 30 mins. Use ROVER to survey GCP’s and you’ll have relative accuracy

Or you can PPK the base coordinates. Let the base collect raw data for an hour. Download the logs from the base. Download CORS-data from your local CORS-station and post process the base coordinates. Then manually enter the coordinates to your Reach RS base. After that you can survey the points using ROVER and you’ll have absolute accuracy

Or you can set up your base over a known point to get absolute accuracy

If I were you I’d read the docs and tutorials from emlids homepage



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Hello Jaakko:

I have the same question here, but using 2 Reach RS2 units, one as a base and the other as a rover.  Is the process of PPKing the base unit coordinates also necessary using Trimble or Leica units?