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Anyone used PPK to establish GCP coordinates?

Dec 15, 2017
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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!
Jul 14, 2017
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Ok so this is diving into the surveying realm which might be why no one has any insight as even some surveyors dont really get post processing... so first a brief, simplified run down on post processing (PPK)...you use your GPS receiver to log a static observation file (which is just a record of signals it is receiving from GPS satellites) which calculates a rough location, then you use that file to compare that location data with the location data of a known CORS station which then calculates precise coordinates for your receivers location aka your GCPs. Hopefully that wasnt to confusing.

Now I'm not familiar with the emlid systems but I know they log rinex data files so you can use OPUS to post process your data with CORS stations through the NGS site. Here's a link to the page you'll want to read up on. OPUS: the Online Positioning User Service, process your GNSS data in the National Spatial Reference System Being a land surveyor I've used OPUS for years and always get great results as long as my data is good to begin with. There are a few keys to getting good data for PPK...
One is to log each point as long as possible, minimum is 15 minutes but over 30 minutes is advisable with 90 minutes being seen by many in my industry to be the minimum for real survey grade accuracy, thats probably why the propeller aeropoints say they need 90 minute observations.
Second is number of satellites and clear sky. You don't want to be next to any buildings, trees or anything else that blocks your receivers view of the sky. In your receivers settings you'll find an "elevation cutoff mask" this should be set at a minimum of 10° with 20-30° being optimal if the number of viewable satellites allows.
Third is receiver set up. Your receiver must be perfectly plumb over your GCP and not move for the duration of your observation and you must get an accurate height measurment from the GCP to the receiver. I always recommend using the receiver on a range/prism pole with a thumb tripod over s traditional tripod since using a range pole means you always have the same receiver height snd never have to wprry about writing it down and keeping track of which height is at which point.

The biggest drawback of using a single receiver is that you'll have to get an observation on every point for a minimum of 30 minutes if you want good data, that means at least 5 hours on site for 10 GCPs. Getting a second reciver would mean you could do use one as a base and collect an observation with it while you use the second to set all your GCPs with somehting like a 1 minute observation for each and then correct them all together. That means you could be on site less than an hour.
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Dec 29, 2017
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@Jesseg138 I agree. Not sure of the price of the PPK systems, but I would think renting a base and rover would be the way to go for establishing GCPs and while your at it get some check shots and hit the points needed to tie into whatever existing networks local networks are being used on site/planning.

Personally I think the best course of action if you want "survey grade" drone data is to have a PSM do all the ground control and let that surveyor do things however he wants since hes the professional and then use the values from that work. If you set your GCPs properly with IRCs or PKs you shouldn't need to re-establish. Just confirm if there are any busts.

Based on the way license requirements work i think a contracted PSM and a contracted remote pilot is the best combo. Who creates the deliverable is up to contract arrangement, but if a surveyor is stamping it I would assume they want to control the data and do the processing in house. just my 2 cents....

Obviously an onboard RTK and accurate attitude / motion correction would change the game, but that doesn't seem to be the norm yet....

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