What is the age interpretation of aerial photos?

(age grade explanation of aerial photos)

(Ma Jianwei)

Determination of stand age groups by aerial photographs. There are visual inspection, regression and image density analysis. Visual method is to determine the age by visual inspection in stereoscopic images according to the age interpretation factors. The regression method takes the height and crown width of the tree related to the actual age as independent variables and the age as dependent variable to determine the age. Image density analysis is to measure the micro-density curve on the photo with a scanning micro-density meter, and determine the age according to its waveform, average value and spatial frequency.

Visual interpretation of age groups is a common method. First, we should be familiar with the biological characteristics, distribution and growth of tree species under different site conditions in the interpretation area, and then interpret the age groups of different tree species in the photos according to the age interpretation factors. The main explanatory factors of age group are the shape, size, height, hue and identifiability of crown. These factors change with the growth of forest age. The same tree species has obvious differences in different age stages. Generally, the age of the same tree species is divided into three age groups in forest interpretation. ① Young forest interpretation: On the large-scale photos, the image particles of natural young forest crown are smaller, the canopy density is larger, the gap between crowns is smaller, the image particles are more uniform and dense, and the stereoscopic image height is very low. In small and medium-scale photos, the young forest does not show the particles of a single crown, and the particles are not obvious in stereoscopic image observation. ② Interpretation of middle-aged forest: On the same large-scale photos as young forest, the image particles of middle-aged forest canopy and the gap between canopies are larger than those of young forest, and the image particles are sparse. The canopy density is smaller than that of young forest, and the stereoscopic image is higher. On the scale of 1∶25000, the middle-aged forest can display the single crown image, and the height of the image can be clearly seen in three-dimensional observation. ③ Interpretation of mature forest: On the same large-scale photo as young and middle-aged forest, the gap between the image particles of mature forest and the crown is larger than that of young and middle-aged forest, and the image particles are more sparse. Image particles are very obvious in stereoscopic observation. On the photo with the scale of 1∶25000, the image particles of single crown can be clearly distinguished. For example, when larch is young, the crown is very small, and when the canopy is large, the crown is connected and cannot be distinguished. In middle age, the crowns of individual trees in the forest are conical and can be distinguished; You can clearly see the boundary line from the light receiving surface of the crown to the backlight surface; The canopy gap is large, and the color tone of panchromatic photos taken in summer is dark gray; Looking at the forest under a stereoscope, you can see half the height of the tree. When mature, the canopy is separated from each other and has different sizes; The top of the tree is flat and the crown outline is serrated; The boundary from the light receiving surface to the backlight surface is not obvious; The gaps between tree crowns vary in size and shape; The height difference of trees is not obvious in stereo image, but the crown of mature forest can be clearly seen in 1∶25000 image, and the image particles are uniform, and the whole piece is light gray or dark gray.

The following points should be paid attention to when explaining forest age: ① The relationship between site conditions of forest growth and forest age. Due to the different site conditions, the image height and particle size of the same tree species at the same age are significantly different. ② The relationship between tree species and age. Different tree species of the same age have great differences in image size and color tone. ③ The change of local scale causes the difference of image particle size.