Pinnacle Template

Mapping Knowledge

What are aerial photographs?

Aerial photographs are taken at an elevated position above the subject to understanding the spatial relationships of any features within a landscape. Aerial photographs can be distinguished as either vertical or oblique views.

Vertical aerial photographs

Vertical aerial photographs are generally taken from a fixed camera in the aircraft's floor.

The camera is fixed with the lens lying parallel to the ground. Thus the vertical aerial photograph will be a near-plan view of the landscape below the camera lens. The distortion of perspective will increase with distance from the centre of the photograph. Vertical photographs are often taken in stereo pairs - the coverage of each photographic frame overlaps a portion of that of the preceding frame. When such pairs are viewed through a stereoscope they produce the impression of a three-dimensional image.

Oblique aerial photographs

Photographs from hand-held cameras generally result in an oblique view. Vertical or near-vertical photographs can be achieved in this way but require aircraft manoeuvres that are not easily sustained or controlled.

Most photographs from hand-held cameras are taken at an angle of far less than 90 degrees to the ground. The obliquity of the photograph depends on the height of the aircraft above ground and the ground distance to the subject. The degree of obliquity determines the degree of perspective distortion. Oblique photographs can also be taken in stereo pairs.

Advantages of Aerial Photography over Ground-Based Observation

Aerial photography offers an improved vantage point.
Aerial photography has the capability to stop action.
It provides a permanent recording.
It has broader spectral sensitivity than the human eye.
It has better spatial resolution and geometric fidelity than many ground-based sensing methods.

Second Generation Orthos: Should I consider?

Second-generation digital orthophoto imagery is becoming popular throughout the GIS industry as cost-effective, timely and able to retain accuracy.

Cost-Effective

Eliminates costs of:

1. Ground survey mobilization and GPS capture

2. Manual Aerial Triangulation methods

3. DTM/DEM capture and processing

Can fly a project at 30% sidelap and 30% overlap if stereo pairs are not desired.

Other Benefits

• Shave one to three months of project duration

• Easily incorporate areas that need updating

• Preserves initial investment of mapping previously done

Retains Accuracy

• First-generation digital orthophoto imagery must meet or exceed the desired accuracy for second-generation digital orthophoto imagery

• First-generation digital orthophoto imagery resolution needs to be equal to or

higher than the second-generation to meet the same accuracy as the firstgeneration digital orthophoto imagery

• DEM/DTM must meet the horizontal accuracy for the second-generation digital orthophoto imagery

• Scans are produced at adequate resolution to meet the requirements of the secondgeneration digital orthophoto imagery

• For the single photo resection approach, should capture 9 to 16 evenly distributed photo identifiable points for every exposure

• Capture photo identifiable points that tie to adjacent images to provide for an accurate block adjustment (AT) Customer needs are paramount, and must be taken into consideration, but in many cases, a second-generation ortho product ideally suits their needs.

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