PrintFor most photogrammetric mapping purposes, the goal is to image as much of the actual ground surface as possible. For this reason, it is customary to fly projects when deciduous trees are without leaves, when the ground is clear of snow and ice, when lakes and streams are within their normal banks, and when the sun is high overhead, minimizing shadows. In many parts of the world, this limits the optimal season for mapping to early spring, when days are getting long, but trees are relatively bare. Add the need for cloud-free skies to all these other requirements, and you can see why the flight operations of most photogrammetric mapping companies are not a big profit center. For any given location, there are a relatively small number of days per year when all the conditions are right for image acquisition.
By the way, these requirements are no different for space-based image acquisition. You already know that the orbits of passive imaging satellites are designed to follow local noon. That takes care of the time of day requirement. Add in all the other requirements, plus the constraints of orbital parameters and revisit times, and you can easily see why large-scale state and county mapping projects are still accomplished with aircraft. I'm sure we'll eventually see seamless coverage of states and nations with large-scale (1 meter/pixel GSD or better) satellite imagery, but it won't all be taken in the same season or even in the same year. Not until there are many, many more high-resolution imaging satellites circling the globe.
Sample Aerial Photography Specifications from Indiana Department of Transportation
Aerial Imagery Guidelines from URISA Quick Study
Several key computations related to flight planning are identified in these documents. These are:
- flight altitude above the ground required to achieve specified photo scale
- number of flightlines required to cover project area
- distance between (and total number of) flightlines required to achieve specified sidelap
- distance between (and total number of) successive exposures to achieve specified overlap
- total number of exposures required to complete project
In most aerial photography contracts, the aerial survey company provides a flight plan, based on customer specifications for scale and resolution, that enumerates all of the quantities outlined above along with a cost estimate based on those quantities.