In this lecture, we'll explore the differences in the outputs generated when the images are processed
in Pix4D with and without the GCPs.
Let�s now measure on QGIS the length of this wall of this structure for the orthomosaic that was generated
with GCP processing.
The length is 3.037 meters.
That's exactly the same as we got on site.
Now, let�s measure the length on the orthomosaic that was generated without GCP processing.
The length is 3.036 meters.
That's just 1 mm off. Clearly, the difference in the relative measurements within the maps is
insignificant.
Hence as I have already mentioned in the previous lectures, if your project needs only relative accuracy
you do not need to capture and process ground control points.
But if you also need global accuracy, ground control points are vital. Next let's look at the global shift
the outputs processed with and without GCPs.
For this, I have opened both the orthomosaic images as layers.
Let�s look at the GCP target L2 to observe the shift.
I will increase the transparency in one of the layers to be able to view both of them together.
And there you go..a clear shift in global position.
Let�s measure the distance between the centres.
It is 1.642 meters.
Now, let�s take a look at the Digital Surface Models (DSMs) of both the projects. The elevation variation
spread looks similar for both the DSMs.
However, the elevation range values are different for the DSM with GCP processing and without GCP processing.
This is because the elevation of points above mean sea level recorded in the images captured by the
drone are not very accurate.
The GCP capture process negates this inaccuracy.
Now let's take a look at the contours of both the outputs.
I want to see if the pattern looks similar.
Yes, they definitely do.
So, it's clear that just the elevation range values are different and not the pattern of variations in
elevations.
Before you do any project, ask your customer if they require mean sea level values with absolute accuracy
or if relative accuracy in the elevation variations is sufficient.
This is another aspect to consider before deciding if you want to use GCPs or not.