Controls Points and Their Types: Ground Control Points, Minor Control Points (Or Pass Points or Wing Points) (Especially for GATE-Geospatial 2022)

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The practical application of the principles of photogrammetry in topographic mapping results in the development of instruments at varying degrees of sophistication for planimetric plotting and/or contouring.

  • But before the instruments can be used for these purposes, it is necessary to establish the image-ground relationships and to join successive strips of photographs together. All these require control points, the provision of which constitutes a major phase in the whole process of photogrammetric mapping.
  • There are different types of control point according to the purposes they are intended for, as follows.

Ground Control Points

  • Control points are required to be set up on the ground to relate the stereomodel and the terrain geometrically. These take the forms of
    • The planimetric control point which is the horizontal position of a point with respect to a horizontal datum (i.e.. . as X- and Y-values in a rectangular coordinate system) and
    • The height control point which is the height of a point with respect to a vertical datum (i.e.. . value in the same system) .
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  • At least two planimetric control points are required to determine the azimuth and scale of the stereomodel, and a minimum of three height control point is needed to allow the stereomodel to be orientated with respect to the height datum for heighting. The whole procedure is known as an absolute orientation.
  • The accuracy of these ground control points determines the resultant accuracy of the map produced from the stereomodel by the machine. Usually, these ground control points have to be actually surveyed in the field, but can be known triangulation stations if a surveying framework already exists. If no such suitable triangulation stations are found within the overlap, the planimetric control points are usually fixed by running a close traverse with a theodolite from a nearby triangulation station or other points of known coordinates, or by resection or intersection or trilateration, whilst the height control points are determined by carrying a line of levels from a point of known height (such as a benchmark) to the desired point with a levelling instrument.
  • It is important that the accuracy of the ground control is compatible with the type of map to be produced, thus, large-scale maps with small contour intervals should have more accurate planimetric and height controls than those for small-scale maps with large contour interval. The accuracy of the ground controls point is therefore determined by the order of ground surveying that has been employed. For general purposes, it is adequate to fix the height control points to an accuracy of ±0.1X contour interval and planimetric control points are fixed such that they can be plotted to an accuracy of ±0.15mm at mapping scale.
  • There are different requirements for planimetric and height control points. For planimetry, the control points should be well defined with sharp contrast against their surroundings such as the junctions of hedges, fences, walls and corners of features such as footpaths; but for height, they should be located in open ground and flat areas of good photographic texture, such as the centres of level road or footpath junctions, and flat roofs of structures. Thus, a single control point will not usually be able to serve both purposes; hence the two sets of control are selected independently.

Minor Control Points (Or Pass Points or Wing Points)

  • These are supplementary controls established on the photographs to serve two purposes:
    • To join the individual models together to form a strip; and
    • For the absolute orientation of each model.
Illustration 2 for Minor_Control_Points_or_Pass_Po …
  • For the first purpose, they must be in the common overlap between two models, i.e.. on three successive photographs, and for the second purpose, they must be chosen close to the edge of the models so that good control in the common tilt is obtained. A third point in the centre of the overlap is also taken.
Minor Control Points (Or Pass Points or Wing Points)

Tie Points

  • These are supplementary control points selected to connect strips of photographs together to form a block. They are therefore located at the centre of the lateral overlap between strips. Usually, one tie point per model is taken, but for lower accuracy, it will be sufficient to have a tie point every alternate model. If very high accuracy is required, two tie points per model should be taken.
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  • In some cases, a minor control-point suitably located at the edge of the photograph can be used also as a tie point. The optimum case arises when a minor control point can be found in six photographs, i.e.. on three successive photographs in one strip and on three successive photographs in the adjacent strip.
  • As these three types of control points have to be accurately identified in the photographs, they are best signalised on the ground before the flight (called Pre-marking) . But this is not always possible, and normally only ground control points are signalised points because fewer of these are required and the highest accuracy needs to be maintained for them. On the other hand, it is also quite common to make use of natural points after flight for ground control points, minor control points and tie points. Natural details such as stones, bushes, road intersections, fence corners, etc which are clearly identifiable in the photographs, are chosen for this purpose. It is important to keep a well-illustrated record of these natural points to help their recovery on the ground or pointing on the model.
  • Since the minor control points and tie points need not be established in the field, they can be selected without regard for ease of access, etc, as in the case of ground control points, but only with regard for their usefulness. All these points, once selected, should be pricked carefully with a pin on the photographs and accurately transferred from one photograph to the other through stereoscopic viewing.

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