 Photo control |
Photogrammetric Processes |
||||||
|
Aerial photography, in all its variants, is an indispensable tool in modern map making both for data-gathering and map compilation; and provides an instant inventory of all visible terrain features in the form of photographic images. These images can not only be interpreted and selected to yield the map detail required, but they can be measured to provide the position and height of that detail.
The stereomodel resulting from the overlapping of 2 adjoining aerial photographs must be fixed in space to the correct scale. To achieve this, control points are needed to ensure that all co-ordinates and heights measured on the stereomodel are correct in relation to the same points on the ground.
Ground control points are essential to relate any result of photogrammetric measurement or plotting (i.e. the stereomodel) to the terrain. A ground control point is a surveyed point which is accurately identified on the photo and whose position is determined in relation to the national triangulation system. Ground control points can either be "Pre-marks", marks made on the ground that will be easily identified on the photo or natural points (e.g. road intersections).
Photo control points are accurately identified points of photographic detail for which co-ordinates have been obtained by photogrammetric methods. When large areas are covered by aerial photography for the purpose of mapping, photo control is obtained by "bridging" a number of stereomodels within a strip of photos, linked to ground control points at intervals along the strip. This process is commonly referred to as aerial triangulation.
Once the stereomodel has been controlled, compilation of detail and heights (including contours) can take place making use of a stereoplotter. After a stereomodel has been set up in a stereoplotter, a "floating mark" seen through each eyepiece appears to be one mark if stereoscopic fusion is obtained. If this "fused" mark is maintained whilst the operator is following detail (e.g. a road) the movements are duplicated on a drawing surface or are captured digitally. The result is, in all aspects, is a true representation of the situation on the ground. By varying the distance between the marks, the floating marks appear to "fuse" at a different height, thus compiling contours is made possible.
By making use of a traditional, or analogue, stereoplotter the resultant detail and contours (heights) are manually plotted on a drawing surface. This accurate "drawing" is then cartographically enhanced to produce the final printed map.
Modern analytical stereoplotters use exactly the same principle, but are linked to computer systems whereby the resultant detail is captured digitally. This digital information can then be used to produce a map digitally by making use of an automated cartographic system, or can be used as input to a Geographic Information System.
| Data Security |
The division: Spatial Information Database (SID) is responsible for the backup/archive and recovery of all digital data that reside on the servers used by the various divisions in the Chief Directorate : Surveys and Mapping.
The backup software in use is the NT backup-, UNIX backup- and ARCSolo software.
The various types of data being secured are:
- Vtrack and Vexcel data - features scanned and processed from the range of maps available from the Chief Directorate : Surveys and Mapping.
- LAMPS2 data - for producing small and medium scale maps.
- ReGIS data - digital map data for producing maps and supply to the public.
- Raw DEM data - to be processed for the production of digital elevation models.
- SCOP data - for producing digital elevation models.
- Network Adjustments data - triangulation and GPS network adjustments for the production of x and y co-ordinates for the trigonometrical beacons and town survey marks.
- Aerial triangulation data - network adjustments for photogrammetric control points on the photos in a stereo model for producing maps.
- Placenames data - information on and all the placenames that appear on the map series produced by the Chief Directorate: Surveys and Mapping.
Legal Disclaimer | Revised: 07/03/02 16:04:30
Copyright © 1997 Chief Directorate: Surveys & Mapping
Private Bag X10, Mowbray 7705. Tel. +27-(0)21-658 4300
(Dept of Land Affairs, Republic of South Africa)