It is very common when mapping a large area that contains structures to forget to account for the added height of the building when planning for overlap.
When flying low, it is very easy to forget that areas that are above the ground will get less overlap due to the wide field of view of most aerial cameras.
If you are flying at 50 meters above ground level and there is a building that is 15 meters tall, you are going to have a bad time. This why the Data Collection guidelines recommend flying at least 4-5 times the height of the tallest object. The goal is to limit differences in coverage due to differences in height.
Example
The example below shows the overlap being set around 70% so the coverage on the ground is OK but the roofs of the structures in the middle get very little coverage. In the case of a shiny metal or featureless roof, this can make it hard to accurately reconstruct. The areas with only 4X overlap ended up being distorted with swirls.
Read more about the effect in the "Elevation Difference Overlap Reduction in Structures" section of the Overlap Reports article.
The Fix: Ground Offset
Most map data collection apps calculate the flight plan to collect overlap on the ground level. This doesn't take the higher overlap into account and the higher areas end up getting less overlap than intended. To address this we had added the Ground Offset feature into Map Pilot Pro. This feature is designed to get the amount of overlap you want on higher (or lower) areas instead of just at ground level.
Other Factors
Rooftops are almost always highly reflective and can be somewhat featureless. This is not a great combination when it comes to the photogrammetric feature matching we employ to reconstruct the scene. We can't do much about the lack of features but measures can be taken to address the reflective nature of roofing materials. Use of a circular polarizing filter attached to your camera will greatly cut down on the reflections from the roof. Also, mapping during overcast conditions can yield better results than doing so during full sunlight.
I have made four maps of my site in Africa. The last three went well. More then one battery. No issues there. So, the last flight of today decided to buy points because we do a large area survey.
The first kmz image I uploaded to Google went well for over 90%. But the other two kmz files are hardly to see. I have downloaded the jpg file and those are perfect. Just not the kmz on GE.
I must say there are a lot of trees and vegetation (the reason for the survey.
For me it's important to put the files in Dropbox so my client have access. But without Google Earth, it has not much use....