History and Development of Mosaics and Photographic Maps (Especially for GATE-Geospatial 2022)

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So much detail is shown by the air photograph that some have advocated the substitution of photographic mosaics and maps for the ordinary line map. A mosaic is made by joining up a series of vertical photographs which have been taken of an area to give a “birds-eye” view of the area photographed.

Problems with Photographic Mosaic

It is not possible to fix the scale of the photographs with precision owing to variations of height and tilts so that when the photographs are joined up, they will give only an approximate representation of the desired scale.

Aerial Photographic Mosaic, Washington, D. C. - Copy 1|Libr …

Since the scale of a photograph is fixed by the ratio of focal length of the camera lens to the height above the ground, the effect of height will be that each contour will be on a different scale. Consequently, no form of photographic representation can be to a definite and uniform scale unless the ground is quite flat.

Techniques of Joining Mosaic

  • In reconnaissance surveys, a mosaic is produced by joining up the photographs with approximate orientation, which ensures that all the area has been covered. This enables a preliminary examination of the area to be made from the mosaic in conjunction with separate prints which have been studied stereoscopically. By this means the topography, geology and vegetation can be studied in connection with engineering projects, and useful information gathered regarding such details as swampy areas, limiting flood lines and geological fault lines.
  • The commercial air survey firms have considerably developed the art of the mosaic. In order to produce a photographic map or controlled mosaic which shall be reasonably near to the desired scale, the photographs are rectified, in printing, for scale and tilt distortions by reference to ground control points. In order to eliminate the effects of height distortions as far as possible, the prints are rectified to scale at the mean height of the area. Photographic maps have occasionally been made with the aid of stereoscopic apparatus with scale correction for each contour. This method is somewhat cumbrous and the additional accuracy is not warranted when the limitations of the use of this type of map are considered.

Example Map from Mosaic

An example of a photographic map and the corresponding line plot is given in below Figures. In this case, the original scale of the mosaic was 1/2,500, each photograph being rectified and enlarged from one taken at a scale of 1/5,000. This illustration is reproduced at a scale of about 1/25,000 from the photographic map which was itself reduced to six inches to one mile (1/10,560) from the mosaic at 1/2,500. The final scale of this illustration is, therefore, ten times smaller than that of the original mosaic, or two and a half times smaller than the photographic map. The line plot was also drawn to a scale of six inches to one mile and is reproduced here at about 1/25,000. In this particular instance the plans are being used by the owner and his estate agent in connection with future developments, and for recording information such as crop rotation, the position of drains and services.

Chilbolton Down
Map Location

Great success has been achieved by the companies in the art of rectification to a required mean scale, and where there are no great variations of height, the accuracy obtainable by scaling from the photograph is often remarkable. Professor S. D. Adshead and Mr R. A. Hudson during the re-planning of Brighton have used rectified enlargements at a scale of 1/2,500 for map revision. In a letter dated 13 May 1936 to the contractors for air photography, Aerofilms Ltd. , they remarked:

“The rectified enlargements to the 1/2,500 scales have proved extraordinarily accurate for the purpose of Ordnance Map revision case arose in which it was necessary to check measurements from the ′ : photographs with measurements on the ground, and the difference; between the measurements was negligible on this scale: for example street measured on the ground was found to be 1,201 feet in length and on the photograph was 1,200 feet.”

Using Separate Photographs in Conjunction with a Mosaic

Separate photographs used in conjunction with a mosaic or rectified photographic map, and a corresponding line plot will give very complete information about an area for all sorts of purposes. The wealth of detail shown on a mosaic is invaluable for all kinds of engineering and economic activity but it is very difficult to “see the wood for the trees” even when the mosaic is not reduced in size as in the example shown. The substitution of unfamiliar objects for the usual conventional signs leads to confusion among the uninitiated, especially if stereoscopic examination of prints is not employed. Although the study of a mosaic and separate prints will in the case of a route location leave no doubt as to the best route, the excess of detail is so great that it is only on a line map containing nothing but essential information that the proposed route can be convenient; laid out. Again, it is only on small scales that the air photograph can provide all the topographical information. The ground measurement which is certain to be required for large scales are better plotted or a line map together with the detail plotted from air photographs. It has, therefore, become commonplace to produce a mosaic as well as a line map for surveys on medium and large scales. There seems to be no limit to the use of the photograph.

In Canada at least twenty uses have been found for photographs other than those for which they were taken. Robbins points out that the mosaic is of great uses in the case of companies operating far away from, say, their London office. The possession of such mosaics and prints by both offices has often assisted rapid decisions to be made with little discussion.

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