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How To Make Ground Control Points

I decided to make some ground control points (GCP). I was able to find some cheap vinyl flooring with a black and white checkerboard pattern.

I bought the vinyl flooring at my local Lowe’s store, named “Tarkett Berkshire 6-ft W Black and White Tile Low-Gloss Finis Sheet Vinyl.” It was $5.94 per linear foot or $0.99 per square foot (six feet wide). Any type of black and white checkerboard pattern would work if the squares are the right size and it’s not too glossy. You don’t want it reflecting the light. The vinyl flooring I bought is not quite flat, which would be best, but it’s not so reflective that it might be a problem. (Or so I hope, I haven’t used them yet.)

The GCP consists of two black squares and two white squares, each six inches square to make one GCP at one square foot. It should be the perfect size if you’re not flying too high. I glued the vinyl floor to some scrap half inch plywood I had that I cut into one foot squares. I used General Use Multi-Floor Adhesive, standard stuff for vinyl floors. That was only $4.98 for a quart. Don’t get it on too thick, less is best.

I’ll also drill two holes in the corner of the white sections for nails. I don’t want the GCP to shift on me, so I will nail down the GCP for the time that it is needed in the field.

I stacked the GCPs on top of each other sandwiched between a sheet of newspaper with weight on top for 12 hours as they dried. Be careful if you do this, I somehow had a couple of them shift on me.

With the glue and spreader, I was able to make 18 GCPs for just over $25. If you need to buy the plywood, I guess it would be another $20. I used half inch plywood, I suppose I could have used thinner plywood. 18 of them are heavy, but I did want to make the GCPs strong so they would not flex.

Attached are some pictures.

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More pricey but have had reasonable results, painted marks on six throw-down baseball bases (have 5 now due to someone walking off with a lonely painted baseball base).

Grey colors seem to work best to this stuff.

Extremely durable… drive on them, rain on them, just leave them - no big deal. they live outside in the back of the pickup.

Just for what its worth!

 

Baseball bases, hey that’s a great idea! Very durable, though I imagine ten of them could be a little heavy.

As a side note for those not familiar, these Ground Control Points are for those who have access to RTK GPS/GNSS, typically professional surveyors since a rover with a base station is around 20K or more. One could also use a commercial RTK network with a cell signal hot spot as your base station and that would be cheaper, but you’d have to be in range of a cell phone signal and you’d typically have to pay a subscription to access the RTK network. I suppose one could also locate them with a total station on an assumed horizontal and vertical datum. The coordinates for the ground control points should be correct to around a centimeter or two. If you need an accurate topographic survey, the GCPs are needed.

After trying to drill into improving accuracy using Pix4D photogremmetric work-flow, I’ve backed away from the expense and troubles of personally acquiring high fidelity control.

If anyone is getting return on their investment by the addition of GNSS equipment in the Pix4D work-flow, I would love to be a student of yours. And if not…I feel your pain!

Again for what its worth…

Mention in other areas of this forum

Aeropoints:


What would be a nice to have implementation into the processing work-flow is this technique:

https://publiclab.org/notes/jasja/6-13-2012/coded-ground-control-points

 …maybe???

The Aeropoint ground control points may be good for informational type of topographic mapping. But if the mapping is going to be used for any type of design work, how are you going to stake it out? You have no permanent survey control on site. You need to be able to accurately stake out the proposed building, storm water structures, sanitary sewer manholes with inverts, and cut & fills for the dirt.I’m not sure how one could create permanent survey control with these Aeropoints.

Absolutely right Ty!

Most of these missions are RE type. Some pre-builds but mostly periodic site audits and post event assessment.

Lasers are used to tie down everything on site, photos are used more for headshops over-views (geo-orthos).

Are you a BIM user?

Gary

I haven’t done any real BIM work. I’m a licensed surveyor, I do a lot of topographic surveys and very few boundary surveys. Occasionally we do an as-built survey and I guess someone could incorporate that into BIM. To do an accurate topographic survey where it is tied to whatever horizontal and vertical datum you’re in, you do need to use ground control points to make it tight. Surveyors in the United States typically use the state plane coordinate system. With Pix4D, I’d like to specialize in adding architecture to topographic surveys. So I need to get good at also recording the vertical faces of the building.

What is RE?

jan opdecam, can you explain what your gcp’s are made from?

Dear Tommy,

It is a autocad drawing. I have plot it and laminated.

I used double side tape to fixed it on woodden triplex boards.

Grts

Jan Opdecam

So just curious do you all shoot the surface of the target to get X,Y,Z? Run a level or measure elevation from known control?

 

If flying much higher than 150’AGL don’t 12"x12" or even 18"x18" targets end up being too small in your imagery?

 

I’m just curious how others are doing their work? If you have to return to the site with temporary targets how can you be sure you deploy them in the same location? How do you pin them down on asphalt or make sure they aren’t moved/removed? Perhaps that does not matter for your missions.

 

 

Derek,

I use 2" lathes cut in half and create an “L” shape. I survey the intersection of the lathes with a Trimble R10 gps receiver. I used to use panels made from vct tiles bought at Home Depot with an “X” made from duct tape. The tiles always had a reflection from the sun at certain angles, making it very hard to pick the mark. I like the lathes, because I usually leave them in the field and don’t have to collect them after the flight. I’m kind of Lazy!

Your camera resolution and focal length has everything to do with what you can see on the ground at a given height. When I change cameras or lenses, I always do many test from different heights to figure out what accuracy I can get with the setup.

Here is an example with a 16mp camera, 14mm focal length at 275’ AGL: