Photogrammetry is a technique which uses photography to measure the environment. This is achieved through overlapping imagery; where the same site can be seen from two perspectives, it is possible to calculate measurements. Again, this technique is not unique to drone imagery, but there is some good news here. Off-the-shelf software, like Agisoft PhotoScan and SimActive, is plentiful and fairly easy to learn.
The hard part is providing your customer with valuable measurement information. And the harder part is competing with firms that have been offering this service for years now using ground-based systems combined with aircraft. For this, you will need some specialized skills and will need to be certified so that you are recognized. One way to get certification is through the American Society for Photogrammetry and Remote Sensing (ASPRS).
An ASPRS Certified Photogrammetrist is a professional who uses photogrammetric technology to extract measurements and make maps and interpret data from images. The Photogrammetrist is responsible for all phases of mapping and other mensuration requirements, which include planning and supervising survey activities for control, specifying photography or other imagery requirements, managing projects for mapping or other mensuration requirements and interpretation.