Although both maps and aerial photos present a "bird's-eye" view of the earth, aerial photographs are not generally considered maps. Maps are spatially correct representations of the earth's surface; they are directionally and geometrically accurate. Uncorrected aerial photos have radial distortion as well as geometric distortions due to inconsistencies in the attitude of the airplane or camera system.  These limitations to accuracy of an air photo interpretation are sometimes quantifiable in terms of scale, resolution and angular distortion but for the most part the "accuracy" of an air photo interpretation is a function of the experience of the interpreter.  Geometric distortions can be corrected but not always, this too is a function of the experience of the person doing it.

Photogrametry is "the art or science of making measurements from aerial imagery". The geometry of an air photo is fixed by the altitude, focal length and film format. These three numbers, along with measurements of the plane's position, can be used to remove the distortions caused by topography, this is called "ortho rectification". By removing the geometric distortions and the topographic distorations aerial photographs can be used for very precise measurements. 

Basic Elements of Air Photo Interpretation

Novice photo interpreters often encounter difficulties when presented with their first aerial photograph because objects are portrayed from an unfamiliar perspective. It's all a matter or experience, once you become familiar with interpreting aerial photography you will become better at identifying features and recognizing diagnostic patterns. It's simple, the more photos you look at the better photo interpretor you become.

Advantages of Aerial Photography over Ground Based Observation

- offers an improved vantage point
- capability to take a "snap shot" of time
- provides a permanent recording and a historic record of changes
- broader spectral sensitivity than the human eye and much broader field of view
- better spatial resolution and geometric/radiometric fidelity than many ground based sensing methods

Types of Aerial Photography

Ultraviolet Photography

General Advantages of Remote Sensing

- Remote sensing provides a regional view
- Remote sensing provides repetitive looks at the same area
- Remote sensors can "see" a broad portion of the spectrum
- Sensors can focus in on a very specific wavelength range and distinguish subtle differences
- They can also look at a number of wavelengths simultaneously
- Remote sensors often record signals electronically or photographically
- Some remote sensors operate in all seasons, at night, and in bad weather

Why Process Remotely Sensed Data Digitally?

Humans are adept at visually interpreting. We can distinguish millions of colors, several shades of gray, and have a miraculous ability to learn to identify and recognize pattern and form. Our eyes and brain are the most sophisticated remote sensing instruments there are, well beyond what machines can ever conceivably be capable of (even in the 21st century!) but computers do one thing better than we do - the same thing over and over and over again very quickly with flawless repetition.

There are limits to a person's ability to distinguish small differences in color. We are especially limited in our resolution of shades of gray. If data are collected using 256 shades of gray, but an analyst can only distinguish 8-10 (optimistically) of them, a great deal of information is potentially lost. The human interpreter is outpaced by the precision of the data.  Computers, however, have no trouble distinguishing 256 shades of gray, each one is individually recognizable.  Whether or not the results are accurate is solely a function of the input if the processing is entirely understood (which is some cases it isn't)

Think of "precision" as the number of decimal places and "accuracy" as how well the results match the real world (what ever that is!) and in this class it is more important to be accurate with your reasoning rather than precisely describe specifics. The activities of human interpretations can be highly subjective, hence, not perfectly repeatable. Conversely, results generated by computer programs - even when they're wrong - are perfectly repeatable.