Fix dimension ground control point or premark is better than postmark such as corner of building or a street corner. For orange item i mean “terpal”. It is plastic material with dimension 20 cm x 150 cm. I am using 18 Mp camera resolution, flying altitude 300 meter, there is no problem to find the premark on image. Look at the picture below.
The important one if we will capture image using UAV we need a fix boundary. Why? First, without boundary we can not define an efficient flight plan. Second, we can plan how many and how we placed GCP on the area. In above picture you can see that i effort to do a normal distribution GCP spreading in all area. the reason is small format aerial mapping contain lots of distortion. So this is the best way to decrease distortion to reach a good accuracy and precision.
How about the picture beside? It seems look good distribution. But this is not good placement for GCP. Why? I have told that aerial mapping using UAV contains lots of distortion, especially in edge of area. GCP placement in the center looks like grid method. Grid method is good for best placement, but all of GCP in the center without any on edge will increase barrel distortion. How about mount of GCP? In my opinion GCP contains of Easting, northing, and elevation. So there is 4 parameters, i will add one GCP for more measurement. For small area i thing 5 GCPs is good. But if you have anytime to measure, adding 2 or 3 GCPs is better.
How do corridor area GCPs placement? Please look at the picture below.
For corridor area, for example like a road, road plan, or anything else, we need at least 3 flight line. In my experience using 3 line GCP looks like picture giving me better accuracy. This method just another way to increase UAV aerial mapping accuracy.
May be useful.