H. Orthophotos
1. General
A map has a constant scale, an aerial photo rarely does. As discussed previously, photo scale is a function of elevation, Figure H-1.
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| Figure H-1 Variable Photo Scales |
For a photo to have a uniform scale throughout requires that all its image points be at the same elevation, Figure H-2.
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| Figure H-2 Constant Photo Scale |
Essentially flat terrain has a uniform photo scale - at the terrain's elevation. Higher and lower flat terrain have different uniform scales. Elevation still plays a part with photo scale. Even with flat terrain at a specific elevation, there's still the issue of relief displacement.
2. Relief Displacement
A traditional paper, or contemporary digital, map has a constant horizontal scale throughout. Planimetric information is elevation-free making scale elevation-free.
That's not the case for an aerial photo because of its optical geometry, Figure H-3.
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| Figure H-3 Elevation Dependent Scale |
Recall that relief displacement, caused by height difference, is radial from the photo's principal point. The amount of displacement is a function of height difference and principal point proximity, Figure H-4.
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| (a) Object | (b) Photo images |
| Figure H-4 Relief Displacement |
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To convert a photo to a map, its images must all be projected vertically to datum, Figure H-5. This draws them in radially toward the principal point.
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Figure H-6 shows the effect of projecting objects in Figure H-5 to datum (positions in black).
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| Figure H-6 Projected Positions |
Because an elevation is a height, ground points projected to datum are also shifted radially. Some straight objects, such as roads, powerlines, etc, may appear curved on the photo depending on their elevation profile and orientation.
The radial shift can be determined using Equation H-1
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Equation H-1 |
Distances r and d are shown on Figure H-7.
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| Figure H-7 Radial Shift |
3. The Orthoimage
An orthophoto is an aerial photo projected to datum; orthoimage is a generic term referring to either a photo or digital product. Why ortho? It's short for orthographic which means "at right angles to": a map, for example, is everywhere perpendicular to vertical lines. An orthoimage would be the same.
The processes to create a physical orthoimage are beyond the scope of this Topic. What is important to understand, regardless the process used, is that some compromises are made due to cost or process limitations. The most visible are buildings that lean, like radially displaced images on a regular photo. Figure H-8 is a digital orthoimage created using image processing software.
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| Figure H-8 Orthoimage |
The orthoimage terrain has been projected to datum but some vertical sides are still visible and building tops are offset from their footprints. This is a cost trade-off: the amonut of processing necessary to remove building height displacement increases orthoimage project costs considerably. Everything else in the orthoimage is planimetrically correct.
Many local GIS use digital orthoimage base maps. In dense residential areas with narrow parcels, Figure H-9, it may appear that buildings cross lot lines.
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| Figure H-9 Boundary Encroachments? |
The apparent encroachment is strictly visual because building height relief displacements were not removed in the orthoimage creation process. The building footprints are in their correct locations, even if not fully visible. Users unfamiliar with aerial image optical geometry and relief displacement characteristics may find orthoimages confusing and seemingly contradictory with other combined data.