AIR PHOTO INTERPRETATION (API)
Air photo interpretation is the art and science of identifying features shown on an air photo and recording valuation features onto the topographic sketch.
Air photos can be bought in consecutive pairs to enable the valuer to view the property through a stereoscope that is, in three dimension. Stereoscopy is found by arranging the pair of photos so that one is viewed with the left eye and the other with the right eye. The eye base of the observer is then extended to the distance between successive photos known as the air base. This may be 2 or more kilometres.
See diagram below:
An air photo run overlaps the next run and the photos in the run overlap
each other. Therefore, there are two overlaps:
The "forward overlap" of about 60.0%
The "lateral overlap" of about 30.0%.
A pocket stereoscope is very useful for use in the field whereas a larger mirror stereoscope is for use in the office:
AIR PHOTO INTERPRETATION - FACTORS
The overlap allows stereoscopy and the alignment of the subject property near the centre of the photo. Once the property is viewed in 3D air photo interpretation can be carried out using the following interpretation factors:
Size is the most important identification feature in rural valuation as a knowledge of the scale of the photo will allow the rapid calculation of the length of fences and the area of paddocks. Houses and many other buildings appear very similar in a photo but size will often be the clue to identification. For example, the shearing shed is generally, the largest building on a sheep property.
Shadows often provide a profile representation of objects on the ground. For example, an overhead transmission line tower is very difficult to identify directly but it's shadow will show the tower outline. However, the shadow is short as most air photo runs are flown 2 hours of either side of noon to limit shadowing which would obscure detail.
The position of objects in relation to other readily identifiable objects is of great help to the rural valuer. The grouping of sheds around he homestead, together with size and shape may be sufficient to identify those buildings. For example, the machinery shed is usually close to the homestead. Cultivation paddocks next to a creek are indicative of
alluvial soils and uncleared land at the rear of the property is the least valuable land with poor access. The holding yards adjoining the shearing shed.
Normal vision is in colour but older air photos may be in black and white. In this case natural colours are represented by various shades of tone from white to black. Some colours appear black such as water and therefore, rural dams are easily recognised as they show up as black triangles against the lighter tones of the surrounding grass or pasture.
Tree canopy cover appears as a dark tone and is an indicator of tree type. For example, pine trees appear black against the lighter tones of the eucalypts.
Texture is the effect of tonal repetitions in an object or groups of objects which in themselves are too small to be discerned as individual objects. For example, wheat and other crops in paddocks, leaves of trees and ploughed land.
The patterns formed by like objects are of great assistance such as in the identification of orchards and vineyards. The patterns may be natural or manmade.
Shape is more difficult than size as most people are unfamiliar with the vertical or plan view of many objects. However, with practice the valuer will be able to interpret features according to shape such as buildings, type of timber (trees) and the shape of land forms.
AIR PHOTO INTERPRETATION STAGES
LOCATE THE PROPERTY ON THE PHOTO
This should be done in conjunction with a parish or topographical map. Once the property is identified the outline of the property can be permanently outlined using a needle and colouring (eg felt pen) or temporarily outlined using a chinagraph pencil. Chinagraph marks can be easily removed with a soft tissue.
MARK RELEVANT FEATURES
A number of chinagraph colours are available so that different colours can be used for soil types, topography, buildings, vegetation. The marked information can be easily modified and changed in the field.
TRANSPOSING ONTO THE TOPOGRAPHICAL SKETCH
The marked details are transposed onto the topographical sketch by eye or by using proportional dividers. This is done with a soft pencil such as HB or B.
DATA CHECKED IN THE FIELD
It is necessary to check the air photo data in the field because:
It provides important feedback to the valuer about the accuracy of his/her interpretation
It is a check for features which have been removed since the photo was taken
It is a check for any new features since the photo was taken.
SCALING AND MEASUREMENT
The scale of the photo can be found using data on the information panel at the bottom of the photo. Modern photos produced at map scales (eg 1:40000) but the scale varies from the centre of the photo to the edges, with different topographic features and the tilt (if any) of the plane.
Therefore, when scaling distances the measurements should be takenas near the centre of the photo as possible. The subject of measurement is shown on 3 consecutive photos in a run so that the middle photo will be the most suitable. For older photos the scale of the air photo is found with the following formula:
S = FL/H
S = scale as a RF
FL = focal length of the camera in mm
H = height of the aircraft above ground level in mm.
All this information is contained in the panel surrounding the photo.
FL = 152.8 mm H = 4 828 032 mm S = 4 828 032 / 152.8 = 31 597
Therefore, the RF for the central part of the photo is 1:31 597. To convert a measured length on the photo to a topographical sketch of for example, 1:10 000 multiply the length on the photo by 31 597/10 000 = 3.1597 or more easily, use proportional dividers. H is the height above ground level but the height shown on the photo is above sea level (ASL).
Therefore, to adjust that height for height above ground level it is necessary to subtract the height of the ground level above sea level.
For the above example, 4 828 032 mm was the height of the aircraft above sea level (ASL) and the ground level is 914 000mm ASL then H = 4 828 032 - 914 000 = 3914 032.
Therefore, the true RF = 1:25 615.
Special photography includes:
Infra red photography measures the heat output from the ground features.
The identification of trees, to determine the health of vegetation, and to find waterlines.
False colour is used to highlight certain features by using colour filters on the camera. This will allow easier identification of a colour on the ground highlighted by the filter.
See air photos