Module 2 - Visual Interpretation

In this lab, we learned to interpret figures in aerial photos using several criteria: tone, texture, shape and size, shadow, pattern, and association. We also compared true color vs. false color (near-infrared) imagery.

First, we wanted to identify features on a photo based on tone and texture. I used the drawing tool in ArcMap to create polygons that enclosed areas of different tones, ranging from Very Dark to Very Light. I then created polygons enclosing areas of different textures, ranging from Very Coarse to Very Fine. The objective here was to determine what type of feature would show up as what tone or texture. For example, a group of houses displays as a very coarse texture as they are spread out, where water is a very fine texture. Below is my map layout identifying features based on tone and texture.

Next we wanted to identify features based on four criteria: shape and size, shadow, pattern, and association. Shape and size define what an object looks like, such as in my map, one of the buildings labeled "looks like" a house. Shadows are an interesting resource when identifying features. They can be a help or hindrance. Some can hinder where they block out other features, but they are often invaluable in determining the extent or height of a tall object, especially if it's narrow and hard to see otherwise. Pattern is used to identify groups of objects that seem insignificant individually but make up a larger feature. One good example is if you are attempting to identify farmland based on crops grown there. Association is a tricky criterion. This is used when a feature could have a variety of uses, but you identify it or narrow down the options by associating it with something nearby. There are examples of all these criteria used to identify features in the map below.

Finally, we wanted to compare a true color photo to a false color or near-infrared (NIR) photo. I examined features that I could identify by their color in the true color photo and compared the color to that of the NIR photo. The most obvious difference was in the vegetation. They appear green in the true color and red in the NIR photo because plants reflect more NIR radiation. This is seen clearly when looking at the mixed pine forests southwest of the river. The fairways and greens of the golf course also show this clearly. Clear water appears black but water with sediment in it appears blue in the NIR photo because it is reflecting visible light, which explains why the river and marshland are blue and bluish-green in the NIR photo. The concrete bridge and sandy, bare earth areas don't look much different between the two photos.

This was a really interesting lab, and more challenging than it appears, as I do not have a lot of practice identifying features from aerial photography. It is a good skill to have though. I look forward to the next module.