DIGITAL ORTHOPHOTOGRAPH: (a.k.a. DOP, DOQ, DOQQ, orthophoto, ortho) a scanned image of a perspective aerial photograph that has been differentially rectified so that ground position displacements due to relief of terrain and tilt are removed. The resulting digital image is scale-accurate and can thus be used as a map, within some limit of accuracy, to measure distances, angles, and areas.
- What are digital orthophotos?
- What are the characteristics of digital orthophotos?
- Why are digital orthophotos useful?
- How are digital orthophotos made?
What are digital orthophotos?
The term "digital orthophoto" (DOP) can be broken down into three parts:
Photo:
An image captured by a camera; in this case, a large camera built specifically
for aerial photography and typically yielding images 9" x 9" in size.
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Digital: An image in the form of a large matrix (grid) of cells, each of which has a gray level (or color). Commonly, a DOP is made from a scanned aerial photograph that has a grid size of at least 9,000 x 9,000 cells. Usually, there are 256 possible gray levels. The resulting computer file is quite large!
Ortho:
This means perpendicular. The scanned photo image is adjusted geometrically
to make
it look like it would if every point on the ground were reduced to a flat
surface as viewed from
directly above. The adjustment process is called
differential rectification
and is based on a computerized model of the terrain and a set of geodetic
control points.
Imagine riding in a low flying airplane over relatively flat terrain, and comparing objects directly below (such as cars or houses) with similar objects toward the horizon. Clearly, the farther the objects are from the observer (you in the airplane), the smaller they appear and the more they appear to lean.
Imagine how an area directly below, such as a marked out square measuring a quarter mile on a side, will appear larger than the same area marked out toward the horizon. (For that matter, the 40-acre square plot at the horizon will no longer appear as a square.)
Now consider how varying relief will affect how ground features appear. All else equal, objects on higher ground will look larger than similar objects on lower ground. And, when those higher objects are located away from the point directly below (i.e., toward the edge of an aerial photograph), they will appear to lean away from that center point.
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These visual effects are due to perspective, the
appearance to the eye of objects with respect to their relative distance and
positions.
Cameras 'see' and record the world the same way our eyes do
with respect to perspective. Therefore an aerial photograph will show
objects diminishing in size the farther the distance from the lens.
An important consequence of perspective is that aerial photos do not have a consistent scale (an attribute that makes maps so useful). For example, 1 inch measured on an aerial photo may represent 400 feet at one area of the photo, and 425 feet at another area of the same photo.
And this leads to the problem. Because of the vast amount of information offered in a snapshot, aerial photos make an ideal base layer for GIS study. However, any measurements of distance, area, angles, or positions using an aerial photo will be in error unless the inherent distortions described have been taken into account. The errors introduced become quite significant when using photos taken at low altitude or of terrain with high relief (hills, stream valleys, or buildings, for example).
Errors are further introduced by tilt , where the axis of the camera taking the original aerial photos was positioned off of the perpendicular to the level ground surface.
A digital orthophoto is the solution to this problem. Created by scanning an aerial photo into a computer in raster file format and adjusting the position of the cells, or "pixels", one by one; a DOP has the appearance of the photo from which it was created. All natural and cultural features are 'shifted' into their correct ground positions, as they would be represented on a map. Because of this scale-accurate quality of DOPs, they have become valuable tools, especially when used together with other data layers in a geographic information system (GIS).
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What are the characteristics of digital orthophotos?
Digital orthophotos combine the image characteristics of a photo with geometric qualities of a map. The scale on DOPs is maintained, creating a good tool for measurement and overlay. The scale of the orthophoto is chosen depending on the nature of the project and the specific needs of the organization. On the county level, maps with scales of 1:12000 or larger are typically produced to give a detailed view of the region. Larger scales are used more for city projects than statewide ones because of the vast amount of data involved and the need for higher accuracy at the municipal level.
Although scale is one of the most important aspects of DOPs, there are other characteristics to be concerned with as well. The resolution (pixel size) of the orthophotograph controls what features are seen and the accuracy of the orthophoto. For example, a resolution of 1 meter means that each pixel roughly represents a square on the ground that is 1 meter on a side. This is the standard resolution for digital orthophotos produced by the USGS.
Standard digital orthophotos are either black-and-white or color-infrared image maps, depending on the nature of the project and the file size desired by the user. Color orthophotography is advantageous because it's easier to understand and provides more data for analysis, but is much more expensive to produce than black and white imagery. Digital color imagery also leads to three times the data volume. Black-and-white orthophotography is suitable for most applications.
Why are digital orthophotos useful?
"A picture is worth a thousand words." Why? For one reason, a picture can contain so much information in its limited space. Likewise, an orthophotograph can offer valuable information that may not be captured on any particular map.
Ground features on a map are graphically represented, and therefore a decision is made by its creator as to what to include on the map and what to leave off. This inherent disadvantage of maps is not an issue with DOPs. All surface features that are large enough to be visible per the DOP's resolution are easily recognizable, although they need to be interpreted by the person doing to viewing since those features aren't inherently identified in the DOP. For instance, a road may be obvious on a DOP, but inside the DOP data file, that road is merely a strip of various grayish cells.
Given this quality as geographically and/or visually accurate backdrops of the ground, DOPs serve as excellent base data layers for geographic information system (GIS) applications. And the initiation of a project based upon orthophotos often leads to the development of other layers, such as transportation networks, contours, or hydrography. The production of Digital Elevation Models (DEMs) or Digital Terrain Models (DTMs) is also likely, since they are used to represent the landscape being studied. With the creation of these various layers, an ideal application for analysis can be derived by the use of a GIS. See the Applications module for more information on reference layers.
How are digital orthophotos made?
Differential rectification is the process of making a digital orthophoto out of an aerial photograph. In lay terms, differential rectification involves the altering of feature positions from where they are on the original photo to their correct planimetric positions. This is possible given the photographic information is in a digital format (that is, the image is composed of pixels). Therefore, the first step is to digitize the aerial photo by scanning. Once digital, differential rectification of an image is basically a mathematical function, with the image pixels being manipulated each in turn. Of course, site-specific or ground control information is required for the process to work.
Example of Rectification
Given that terrain elevation variation is a major influence in the distortions on an aerial photo, a DEM or DTM of the represented region is required. The elevation model serves as vertical ground control ; an elevation value can now be assigned to each pixel of the scanned image. 30-meter DEMs of nearly the entire state of Wisconsin are available from the USGS Global Land Information System , which also has 10-m DEMs in limited areas.
Horizontal ground control must also be addressed. Remember the goal is to 'shift' features on the scanned image to their correct ground positions. These positions are conveniently identified by their map coordinates. Given a fair distribution of points on the image whose ground coordinate positions are known control points and assigned in the process, the rectification of the extent of the image can be performed. These control points of known position can be thought of as 'anchors'.
Variance parameters are mathematically evaluated by comparing the control point's relative positions to each other and what those positions should be on a map. These parameters can then be used to determine error in position for each pixel of the entire image.
More information on the production of orthos can be found at the following resources:
- USGS's Digital Orthophoto Quadrangles page
- Making DEMs and Orthophotos
- Orthophoto Production, New Practices and Possibilities
Digital orthophotos can be thought of as 'true-position' photos. Orthos offer the value that aerial photos have by displaying a region of interest without the limitations of map symbolization. And they offer the scale consistency that gives maps their power. As a consequence, distance, area, and angles can be measured directly on the image.
Okay, so orthos are a 'true-position' photo...what are they really good for? For further information on orthophotography uses, please read on in the Applications section. Then if you need assistance in picking the right ortho for your project, see OrthoFinder's Selection page.
In our next section, we take a look at the various producers or Sources of orthophotography in Wisconsin from the federal level down to site-specific and historical orthophoto production...