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Author Topic: Do I need GCPs to get an accurate DEM?  (Read 9643 times)

jmdg125

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Do I need GCPs to get an accurate DEM?
« on: February 12, 2015, 03:39:22 AM »
I'm shooting with a DJI phantom vision 2 plus.  I shoot in RAW so I get GPS coordinates and altitude with each photo. 

My question seems simple: do I need GCP's to ensure accurate elevation and georeferencing? My initial outputs would seem to suggest so, but I'm looking for confirmation.  I'm able to compare the DEM outputs from Agisoft against a high-resolution lidar-based DEM.

The site is under transformation and I'd like to detect small-scale elevation changes.




bigben

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Re: Do I need GCPs to get an accurate DEM?
« Reply #1 on: February 12, 2015, 08:57:23 AM »
The tutorial headings would suggest that:
Quote
This tutorial illustrates how to produce your first georeferenced orthophoto and/or DEM of high quality with Agisoft PhotoScan Pro, providing that you have ground control points (GCPs) data.

ARF

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Re: Do I need GCPs to get an accurate DEM?
« Reply #2 on: February 12, 2015, 11:38:43 AM »
High quality doesn't necessarily mean high accuracy.  You might get a good looking model with lots of detail but without accurate control points you can't reproduce the results reliably.  The problems include not having a benchmark to reference change and you can get issues with vertical tilt and warping across the site.

You really need the ground control unless you use very accurate camera positions (via survey grade RTK GNSS) and optimisation procedures within agisoft.

I'm not sure what your interpretation of small scale change is? But your gps might be a accurate to 2 to 5m. 

I suggest you try flying a site that doesn't change on different days and compare your models.

MiguelVarela

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Re: Do I need GCPs to get an accurate DEM?
« Reply #3 on: February 12, 2015, 01:46:39 PM »
Hi jmdg125,

You mentioned that you have the GPS coordinates from the Phantom, but that is a consumer grade GPS so the accuracy is less than 10 meters. For elevation is even worst.

So the GPS will be ok to georeference your model as a general location of your study area but definitely not a data to be used for accurate elevation measures. For that you need to use either GCPoints or a survey grade RTK with high accuracy.

What accuracy are you trying to get?

Mdaemon

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Re: Do I need GCPs to get an accurate DEM?
« Reply #4 on: February 12, 2015, 07:28:51 PM »
Hi Miguel,

you are right. You need RTK GPS to be below 10 cm deviation.

Martin

jmdg125

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Re: Do I need GCPs to get an accurate DEM?
« Reply #5 on: February 12, 2015, 10:31:09 PM »
Thanks for all the responses.

Ideally, I'd like to detect changes in elevation of <1 foot.  Since the scene is highly dynamic, I'll be shooting frequently.

If you have a chance, please take a look at the attachment.  It shows the scene at two time points.  The red line is meant to indicate - albeit roughly - where elevation is relatively constant.  The dirt formation in the middle is what's changing (sediment from a nearby river is being added to the formation).  The width of the scene is ~280 meters.

So, I think I'm hearing that I need to invest in another piece of equipment that can get accurate measures of X, Y (and Z?) at various locations throughout the site. Is this right? Also, any suggestions on how to lay out the GCPs given the dynamic inner portion of the scene and the consistent edges?

I'm new to all of this, so thanks for bearing with me!

 
« Last Edit: February 13, 2015, 01:22:27 AM by jmdg125 »

ARF

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Re: Do I need GCPs to get an accurate DEM?
« Reply #6 on: February 13, 2015, 01:24:13 PM »
Yes, you should place ground control before flying each time and coordinate the marks accurately using any survey method.  Then process by following the General ground control point tutorial  for help.

jmdg125

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Re: Do I need GCPs to get an accurate DEM?
« Reply #7 on: February 15, 2015, 10:39:30 PM »
Ok, and I need to obtain elevation at each GCP, not just lat/lon - is that correct?

Also, will it significantly warp things if I don't have a GCP in the middle of the scene?

ARF

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Re: Do I need GCPs to get an accurate DEM?
« Reply #8 on: February 16, 2015, 11:49:03 AM »
Correct, accurate elevation on ground control is critical for your application.

I'm not sure what impact not having control in the middle would be but I would definitely place a mark there and coordinate if possible.

Any areas without control is where the camera and model errors will hide.

The first time you do it I suggest you place lots of control and then you will have independent checks points not used in the processing to check the accuracy of your results.

dcm39

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Re: Do I need GCPs to get an accurate DEM?
« Reply #9 on: February 17, 2015, 06:20:27 PM »
Careful survey design is also a really important element of this. While a dense network of GCPs will allow you distort your 3D model back into shape, it is better if you can minimize the camera model distortion in the first place.

Because of the progressive nature of the bundle adjustment adding photos one/two/few at a time, you can get build up of systematic errors across the model.

The most typical case for this is actually planar acquisition - if you take all of your photos at nadir (straight down) then there is a tradeoff between the radial distortion and the focal length for the lens. The net result of this is a "bowling" of the entire model.

You can either fix this after the fact, with lots of ground control, to restore the model to "flat" (I assume this is what the optimize button does in photoscan), or you can minimize it before hand by careful survey design - though ideally you would do both! You need to have a number of off-nadir images included in the dataset - this will significantly reduce the lens miscalibration.

As mentioned above, you will need some precisely located ground control (>=3) in order to get an accurate tip/tilt for your model. Standard GPS (as on the phantom) will give you a reasonable scale for your model if your survey area is large enough (typically ~3m horizontally, which will be 1% over 600m).

As ARF said though, whichever way you do it, you should definitely use plenty of independent checks the first time you do it.

jmdg125

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Re: Do I need GCPs to get an accurate DEM?
« Reply #10 on: February 19, 2015, 08:39:34 PM »
These responses are very helpful, thanks so much.

So, if the cost of an RTK GPS unit is too prohibitive for my project, am I out of luck? 

Or, is there some way to do this without an RTK if I don't need absolute elevation above sea level, but rather am content with assessing relative change in the scene between two time points?  In other words, what if I ditch the hopes of getting this thing geo-referenced, and instead just focus on elevational changes in the scene? I'm assuming I still need a unit of some sort to truth things, but thought I'd check with you all to confirm.


ARF

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Re: Do I need GCPs to get an accurate DEM?
« Reply #11 on: February 19, 2015, 10:23:18 PM »
RTK gps would be the quickest and easiest method but there plenty of other ways to accurately survey the GCP's.  You could find a RTK gps for hire or loan. 

The other alternative is using a total station preferably with an EDM to coordinate the gcp's.  You could hire these from a dealer also.

dcm39

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Re: Do I need GCPs to get an accurate DEM?
« Reply #12 on: February 26, 2015, 03:17:15 PM »
It really depends on how much you need accurate tip-tilts. You can use an accurate scale length to scale the model (e.g. use a tape measure on some object within the scene), and you could use handheld GPS to set a rough georeferencing (good enough for a lot of things).

Alternatively if you're looking at changes through time, you could borrow a survey GPS to measure some control points that wont change over time and just use those repeatedly.