How to use GCP - help

Hi all,

until now I made 3D mapping and nadir photography without any ground control points.

Next week I'll have to map a terrain on a slope, and I would use ground control points to get precise results.

I'm using a DJI Phantom 3 Professional and Pix4D, I can fly between 15 metres up to 100 metres AGL. The area is about 500x150 metres.

How can I print and use GCP to get an accurate survey? Can I measure the distance between GCPs with a yardstick and then apply that distance in Pix4D?

Thanks in advance for anyone that will help me.

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  • Not sure how you want accuracy w/out a surveyor?  I tested my Garmin on spots I had surveyed by a surveyor, the altitude was off by about 10'.  They XY was off by just a few feet.  He spray painted orange on top of fence posts I could pinpoint in photos.  

    Each area uses a different coordinate system that Pix4D provides in their scroll down.  

    Is this a project or a job?  If it's a job, they might have survey points set up already.  

    I work w/ engineers & surveyors, I just collect the data & they give me the coordinates & an engineer (P.E.) has to sign off on it to make it worth anything.  

  • Not so sure how the distance will help you in working out your coordinates.

    Why not survey the GCPs with high precision GPS?

    • High precision GPS is unaffordable for me at the moment.

      My main target is to survey a reliable measurements rather than an accurate GPS position on the map. I apologise if I didn't specify this before.

    • In this case why do you simply use low precision GPS such as hand-held?

    • I can use an iPhone 6 with an app like MotionX GPS to find the best accuracy on the map and copying the GPS coordinates.

      If I should do so, the survey would be more precise?

      Sorry for my beginner questions... :)

    • My understanding is that, the GCP are to improve accuracy rather than precision. If you are using the same camera, I don't see how precision should be a problem. The only things is the data won't be in true world space.

    • Irondome is correct. The software you'll use to process your pictures will most probably give you a spacially accurate job, with all points in the point cloud being well placed relativelly to each other, but placing that job on the face of the earth (georeferencing) will be messy due to the lack of precise positioning of teh GCPs. If you don't use it for mapping, like just to have a vision of some place then it'll be fine, but if you work like you've described as professional mapping, then you'll be embarrased some day if your client finds out.
      Better save than sorrow...

    • Clear, thanks for your support.

      So printing GCPs and place them on the terrain, in my case would be useless unless I use a high precision expensive GPS like Leica to place them georeferenced.


    • Your original statement was about accuracy hence I think most answers were focused into that. And surely if you are doing professional mapping you will need to use GCPs for accuracy. But the GPS you don't have to buy one, you can rent one. I usually hire a guy (surveyor) to pick the points for me. For such a small area you work on, only 3 points should be sufficient. If it is an area you map regularly, you could build permanent markers and have them surveyed once some some $$!

    • Good advices IronDOME, thanks.

      I will look for someone here in my area. It won't be easy since I'm on high mountains, but I'll look for him...

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