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Analogue plotting instruments 
À significant advantage of photographic sensors is that the photographs 
can be used in some existing photogrammetric plotting instruments. 
There are however limitations. Images from non photographic systems 
cannot be used without extensive processing and correction. 
  
The image on a photograph taken from around 250km above the surface of 
the earth will be of the curved surface of the earth and will be subject 
to distortion due to refraction and to any effect of using a camera 
window. It is usually desirable to plot a map on a particular map 
projection and therefore to compensate for the image of the curved earth 
on the photograph by changing x and y parallax or by changing the model co- 
ordinates. Radial compensation of 0.6mm in the corner of a photograph 
is required in the case of a metric camera photograph. The effect of 
refraction will be very small because of the use of a normal angle 
lens. Meier (1974) has investigated the effect of temperature and 
pressure changes across a camera window and distortion may reach 40um 
in any radial direction, however Zeiss (personal communications) expect ; 
the camera window on Spacelab to have no effect because the camera principal 
distance has been changed to compensate. 
In order to use analogue instruments it must be possible to allow for these 
effects in an instrument which will accept the camera principal distance and 
format size. The metric camera and the large format camera have principal 
distances of 305mm and the images can therefore by accommodated in many 
plotting instruments, only the centre of the large format camera image will 
fit into a 230 x 230mm plate holder and, whilst plotting can be carried out 
from such a reduced image, the advantages of the wide angle base:height ratio 
are lost. 
The combined effect of earth curvature, refraction and camera window will 
be to create a distorted model, this may be corrected by analogue devices 
but their precise effect should be investigated before correction is assumed. 
In the absence of any correction,co-ordinates may be recorded in the model 
space in an XYZ system and if ground control co-ordinates are known in a 
geocentric co-ordinates system, a model may be set up and recorded co- 
ordinates transformed to a required projection. 
A typical analogue correction device will apply a displacement radial to 
the nadir thus creating a projection on a plane which is tangent to the 
surface of the earth at the nadir point. If the altitude of the camera was 
250km, this projection has a scale error of 0.9976, 100mm from the nadir 
at photograph scale; when referred to a Transverse Mercator projection on 
which scale is correct along the central meridian, a differential scale 
error is created, which if distributed over the model would be a max imum 
of c.9988. This effect can be reduced by scaling to control over a small 
area. 
The radial correction is normally computed by assuming that the point to 
be considered is on the surface of a sphere. If relief is present this is 
no longer correct and can cause errors in height of 10m with relief of 500m. 
Before using any analogue correction device it is necessary to determine 
the precise effect of the system and to analyse the errors at control points 
before proceding to map compilation. Clearly the distribution of control is 
also critical. |