Best way to acheive Vertical Accuracy


Our clients are into open pit mining, and they are expecting a very high vertical accuracy to match with their own model generated with total station data. Client was told by some expert that Adding 24 GCP’s in one square kilometre area is a solution. But, GCP’s of that many number is useless, when the terrain is not that uneven and it defeats the purpose of using them. I am assuming a bunch of the 24 count are check points not GCP. I would like to know what the most reliable way is to achieve vertical accuracy that we can expect using photogrammetry.

There is no absolute number of GCPs required for a project in order to maintain high accuracy. As you seem to be aware, the number will depend on what the project area is like. Typically, there will be diminishing returns on the number of GCPs. So, simply adding more won’t always increase accuracy values. Without knowing your project, you might only need approximately 10 GCPs with another handful of checkpoints.

When talking about accuracy, much will depend on the resolution of your camera and the resulting ground sampling distance. Typically, the accuracy will be 1-2 times your GSD for horizontal and 1-3 times for vertical. So if your GSD is 3cm then you can expect accuracy in the range of 3-6cm horizontal and 3-9cm vertical.

You need a PPK drone. With the right drone, you can achieve high absolute accuracy without GCPs. Take a look at the WingtraOne.

I am using Mavic 3 Enterprise RTK. Project areas is 2.3 square kms. It’s basically a hilly terrain. How do I generate smooth contours during processing?

What program are you trying to use for contour generation?

I have used Pix4D Survey. The contours info is not reflected in the shape file that i have exported as well in QGIS. Not sure what happened in exporting process.

This is a known issue with Survey. The vertices of the contour lines contain the z-values. So you need to add an elevation field to your attribute table. Then calculate the geometry attributes to extract the elevation data. Unfortunately, this workaround is required.

Hi Mike,

I was told to use Field Calculator to display attributes info of the contours. If i go by the field calculator, i am getting the elevation as an integer only, despite selecting the type as double or float.
The requirement is in decimal form preferably till 3 decimals. How do i get in decimal format. Attached the screenshot of what i am referring to.

Please help me with this.

Hi @rayavarapunani,

The comma is added only if you select a contour interval smaller than 1m in PIX4Dsurvey.
To add decimal format, you would need to refer to the ArcGIS documentation.
Also, note that having a contour line expressed as X.000m does not mean that the accuracy is in the order of millimeters.

I’ve found that as long as my GCPs are within 1000 ft of each other and that you have GCPs representing the lowest (ish) and highest (ish) elevations on the site, the accuracy is consistently good. I have surveyors stamp my data on a regular basis. RTK is great, but I haven’t found it to be as accurate as ground control.


Thanks @Chris_Putnam for the info.

Of course. Good luck!