Scale & Area Measurement
Scale is the ratio of a distance on an aerial photograph to that same distance on the ground in the real world. It can be expressed in unit equivalents like 1 inch = 1,000 feet (or 12,000 inches) or as a dimensionless representative fraction (1/12,000) or as a dimensionless ratio (1:12,000). Photo Scale
Knowledge of the camera focal length and the aircraft altitude makes it possible to determine photo scale (PS) and the representative fraction (RF) of a photo.
1. Scale ratio is also referred to as the proportional scale. 1:20,000 is read as "one to twenty thousand". The scale ratio is always written as one unit on the photo or map to the corresponding number of units on the ground.
The important thing to keep in mind, once you have mastered measuring distances, is that areas have squared units. For a rectangular area its length x width, so if you measure both and convert these distances remember that if you are multiplying them together the resulting units are squared.
Techniques for area measurements: 2) Transect Line intercept or transect method of canopy estimation is analogous to the dot grid method and is similarly accurate. In this method lines are superimposed on the aerial image and the length of each line that overlays tree canopy is compared to the total line length. Canopy cover is then calculated as: % canopy cover = 100 x (length of lines covered by trees/total length of lines in sample)3) Dot Grids Dot grid area estimations involve laying a transparent grid over an area of interest and counting the grid cells or dots that fall within that area. This is a quick and easy way to estimate areas, or to estimate the density of objects, and is relatively easy to understand. Each dot or grid cell is proportional to an area according to the scale of the source image, summing the number of dots or grid cells and multiplying by the scale conversion allows you to estimate areas quickly. The four diagrams below illustrate different methods for estimating area. The scale is fixed for each diagram (1 km). Each of these methods has tradeoffs between precision and accuracy, but all are valid methods of estimating areas.
With the techniques described above it will be possible to make estimates of areas and lengths. This can be useful when interpreting air photos because sometimes relative sizes, and differences in areas, can lend support to an interpretation. When exacting measurements are required however dot grids and scaled measurements need an additional level of correction. The devices for performing this correction are called "stereoplotters". |