The following link shows a sample Keyline Photomap during an early stage of design.
Dam walls, waterlines and some channel lines are shown on a true scale aerial photo. (The photomap may take a few minutes to download. I suggest you start a new browser window. (Ctrl-n does it in MS IE) Click the link and use Alt-Tab to switch back to here and continue reading while it downloads.)
Click here see a section of a Keyline Photomap
A good contour map is an essential resource for property planning. Contour maps can be economically produced from aerial photography. Most of the Australia has already been flown and photographed. The topographical maps available from the various state departments of mapping and survey are produced from overlapping stereo pairs of aerial photographs. Generally the contour interval used on these maps is not as close as is now possible from current photographs.
For property planning purposes it is best to have the contour interval as close as possible. Closer contours, unless they are just interpolated from the existing contours, reveals more detailed information about the land shape.
Keyline Designs employs the latest technology to produce more detailed contour maps. Properties covered by departmental maps showing only ten metre contour intervals, can be re-mapped with good form line contours at two metre intervals.
In flat country, rectified aerial photos are virtually true-to-scale photo maps. After rectification the aerial photograph can be combined with other feature information and used for true to scale planning. The process of photographic rectification adjusts for tilt and scale, which enables the image to be fitted onto an existing plan.
In flat country rectification will virtually produce true-to-scale photo maps. In undulating country it is necessary to process the information further and so reveal the third dimension, which is the height relationships within the landscape.
Even without rectification some aerial photographs can be used as a base layer for a Keyline Designs property plan. The contour data is placed onto the aerial photograph and the property plan done on top of this. The result is a Keyline Photomap, which is a Keyline Design on a contoured aerial photograph.
We produce orthophoto maps by scanning overlapping pairs of aerial photographs into a specialised computer that generates a 3D stereo model of the area. A finely spaced survey grid is draped over the relevant areas within the model. The height of each point on the grid is calculated by computer analysis of the corresponding points in the scanned images. Each point on the grid is then linked to its nearest adjoining points using a process called triangulation. After ‘brakelines’ are inserted the computer interpolates where contour lines intersect the triangulated grid lines. A new contour map of the modelled area is produced by joining the appropriate intersection points. Computer analysis of point heights greatly increases the speed and reduces the cost of map production. Some landscapes are unsuited to full computer analysis and sections of the area may be assessed by more standard procedures.
Most of Australia has been photographed by National Mapping at scale of 1 in 80,000 using a super-wide lens.
In Queensland, the Lands Department have the most of the eastern half of the state covered with at least 1:25,000 scale photography flown at about 4,000 metres (12,500 feet). Adjacent photographs from the same run overlap 60% and at this scale the maximum 3D model is 1,984 ha. (5.75 Km * 3.45 Km = 1,984 ha).
The scale of the diapositive is the quotient of the flying height divided by the camera lens focal length. Focal lengths are usually 154 mm (wide angle) or 88 mm (super-wide).
The number of photographs required depends upon the height and flight path of the camera aircraft.
When four or more models are needed to satisfactorily map a particular area it is time to consider having the area photographed again to minimise the number of models. The best results come from custom quality photography on pre-positioned ground control marks. Lower flying heights can be used and will result in clearer photo-maps, closer contour lines and less 3D models. Commercial aerial photos taken of a particular area can range down from $1,000 to $60 each.
Keyline Designs uses the contoured aerial orthophoto maps and digital data to plan water storages; irrigation areas; road locations; agro forestry development; building sites and property subdivision.
The scale limits both point height accuracy and the grid spacing. A twenty-five metre (25m) grid on the ground requires analysis of a one millimetre (1mm) grid on a 1 in 25,000 scale diapositive. The limit of accuracy for point heights on 1 in 25,000 scale diapositives is about ±0.8 metres. The minimum valid contour intervals equals the span of accuracy. So 1 in 25,000 scale photography is limited to contour form lines at 2m intervals, (theoretically 1.6m intervals).
Greater accuracy requires photography from a lower height. To achieve an accuracy of ± 0.5 m for spot heights requires 1:15,000 scale diapositives. This would need to be flown at about 7,500 feet. Such a photo has a 3.45 Km side, which limits the model to 714 ha. (3.45 * 2.07 Km ).
The best ground control comes from surveying the positions of readily identifiable marks that are placed on the ground prior to the aerial photography. The white "+" signs seen on some roads and footpaths are existing ground control reference marks.
Although the ground control points may be related to the national survey grid and height datum, for a farm surveys the contour heights and grid lines need not be related to anything external to the property.
When using existing photography, survey the relative heights and distances between some points that surround the area and some high and low points within it. An enlarged aerial photo helps in choosing good control points.
If it is not practical to go onto a property to survey the control points for the model, it is possible to obtain orientation data from the State or Commonwealth topographic map that covers the area. Departmental topographic maps are usually under $10 each. Cadastral maps and local Council maps may also provide property plan dimensions. Combining these maps will provide ground control heights and distances for the model.
The accuracy of the resulting 3D stereo model will be limited to that of these maps and some errors of tilt and vertical scale may occur. Never the less very valuable contoured form lines can be produced.
For further assistance in obtaining your needs contact:
P.O. Box 3289 Southport Queensland 4215 AUSTRALIA
Phone: (07) 5591 6281 Fax: (07) 5527 0847 Mobile: 0418 745 120
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