Background Extent of Coverage Acquisition Processing Steps Data Characteristics Spatial Resolution Spectral Range Data Organization Data Availability Procedures for Obtaining Data Products and Services Applications and Related Data Sets References Appendix
The U.S. Geological Survey (USGS) is the lead Federal agency for the collection and distribution of digital cartographic data. The U.S. Department of Agriculture, Agricultural Stabilization and Conservation Service (ASCS); U.S. Department of Agriculture, Soil Conservation Service (SCS) and the USGS cooperate in a digital orthophoto program. The USGS's Earth Science Information Centers (ESICs) distribute digital cartographic and geographic data produced through the USGS's National Mapping Program.
Orthophotos combine the image characteristics of a photograph with the geometric qualities of a map. They serve a variety of purposes, from interim maps to field references for earth science investigations and analysis. The digital orthophoto is useful as a layer of a geographic information system (GIS) and as a tool for revision of digital line graphs and topographic maps.
Unlike a normal aerial photograph, relief displacement in orthophotos has been removed so that all ground features are displayed in their true ground position. This allows for the direct measurement of distance, areas, angles, and positions. Also, an orthophoto displays features that may be omitted or generalized on maps.
The National Aerial Photography Program (NAPP) imagery and NAPP-like photography are the primary sources of imagery used in the production of 1-meter digital orthophotos.
NAPP photography is quarter-quadrangle centered (3.75-minutes of latitude by 3.75-minutes of longitude in geographic extent) and taken at an aircraft altitude of approximately 20,000 feet above mean terrain using a 152 millimeter focal-length camera. The scale of the NAPP photography is approximately 1:40,000. National High Altitude Photography (NHAP) program black-and-white photography (1:80,000 scale) or NHAP-like photography (1:80,000 scale) is used in the production of 2-meter digital orthophoto quadrangles (7.5-minutes of latitude by 7.5-minutes of longitude in geographic extent). Orthophoto quadrangles may also be produced through the mosaicking of digital orthophoto quarter-quadrangles. Color infrared (CIR) photography may be used as a source; however, the resultant image would be an 8-bit, black-and-white digital orthophoto. Although NAPP will be the primary source for imagery, this does not preclude the use of additional aerial photographs or digital images in the future.
The DOQ coverage area includes the conterminous United States, Alaska, Hawaii, and Puerto Rico.
Each aerial photograph is accompanied by calibrated coordinates of the eight fiducial marks which appear on the photograph. These marks define the frame of reference for spatial measurements made from the photograph. This information, along with the calibrated focal length, is generally provided from a camera calibration report. The fiducial marks are captured during the scanning of the photograph and their coordinates are stored in the raster coordinate system.
Ground control refers to a set of points with known positions or elevations, or both, which are used as fixed references in establishing the exact spatial position and orientation of a photograph relative to the ground.
A Digital Elevation Model (DEM) is an array of elevations at regularly spaced intervals. DEM data can be obtained by stereo-profiling aerial photography, as a by-product of orthophoto generation, from the scanning of contour data from topographic maps, and from the conversion of DLG data.
The process of creating a digital orthophotograph begins with the scanning of an aerial photograph (film diapositive) using a high precision microdensitometer with an aperture no greater than 32 microns. The scanner converts the photographic image densities to gray scale values ranging from 0 to 255 for black-and-white photographs. In the case of a color infrared photograph, three separate scans are made and converted to gray scale values, one each for the red, green and blue bands. A separate monochrome image from the scanned CIR photograph scan is also generated at this time.
In the USGS Digital Orthophoto Production System (DOPS), the ground coordinates, camera calibration parameters, and photo coordinates of the of the source photograph are used to extract the exterior orientation parameters through a space resection equation. The resection determines the position and attitude of the image with respect to the exterior coordinate system. After resectioning, the raster image will be in the same coordinate space as the source photograph. Conversion parameters developed through a least squares transformation are used to convert each raster line and sample location into the aerial camera coordinate system.
A similar process occurs to a profile pair of DEM cells. The first two profiles of the DEM are input to create a profile pair. The exterior orientation parameters developed in the resection are used to determine the photo coordinates of the DEM cell corners. The raster image and DEM cell are now related to the same photo-coordinate space and the number of lines of scanned raster data needed to rectify the profile pairs is determined.
Scanned raster coordinates of each DEM cell corner in the profile pair are calculated by applying a linear conformal transformation equation using the photo coordinates of the DEM corners along with the transformation parameters.
The individual 30 meter by 30 meter DEM cell is partitioned into the desired ground resolution. To attain a 1-meter ground resolution for 1:40,000 NAPP, a DEM cell will be divided into 30, one meter by one-meter (900) DEM elevation subdivisions. Each DEM cell is bounded by 4 elevation posts which are used in a linear interpolation in order to establish an elevation for each cell subdivision. The photo coordinates for each cell subdivision are computed using the ground coordinates and previously determined orientation parameters.
For each raster image cell subdivision, a brightness or grey-scale value is obtained using nearest neighbor, bilinear, or cubic convolution resampling of the scanned image. An inverse transformation is then used to relate the image coordinates referenced to the fiducial coordinate space back to scanner coordinate space. This process takes place cell to cell in a profile pair, with each profile pair processed sequentially.
The output from this process is a digital orthophoto, rectified so that each of its pixels are referenced to its proper ground position. The digital file can then be stored on any number of storage media, including CD-ROMs, 9-track and 8mm tapes. Using a film-writer, a continuous tone hard-copy orthophoto can be produced.
The locations of pixels of the scanned image and the elevation profiles of the DEM very rarely coincide. This necessitates an interpolation of the image scan lines to the DEM profiles through a process called resampling. Depending upon which resampling algorithm is used, some systematic occurrence of artifacts may result. Three resampling algorithms may be used during the construction of a digital orthophoto. They are: Nearest-neighbor, Bilinear interpolation, and Cubic convolution.
A step-by-step digital orthophoto creation process flow is detailed in the Appendix area of this Guide.
The resolution of the digital orthophoto is a result of the scanning aperture of the microdensitometer used to capture the digital image. If a scanning aperture of 25 microns is used on a 1:40,000-scale image, the ground (pixel) resolution is one meter. A 7.5-micron scan yields a ground resolution of .3 meters; a 15-micron scan equates to a .6 meter ground resolution. The horizontal ground resolution (x and y components) is one meter for digital orthophoto quadrangles. If digital orthophotos are scanned at a finer resolution than one meter, they will be resampled through a bilinear interpolation process to obtain one meter horizontal resolution.
The geographic extent of the digital orthophoto is equivalent to an orthophoto quarter-quadrangle or quadrangle (3.75- or 7.5-minutes), plus a minimum of 50 meters to 300 meters of overedge, sufficient coverage to encompass the four primary and secondary horizontal datum corner points. The overedge is useful for edgematching and mosaicking of quadrangles by offering areas outside the primary area of interest which facilitate tonal matching between images. Every orthophoto is a rectangle but may not necessarily be the same size as its adjoining neighbor. The normal orientation of the data are by line (rows) and samples (columns).
The maximum bounding rectangle for the digital orthophoto quadrangle or quarter-quadrangle in the conterminous United States and Alaska is 7,700 lines by 7,100 samples. For Hawaii, the maximum bounding rectangle is 7,700 lines by 7,300 samples. The maximum bounding rectangle for Puerto Rico is 7,600 lines by 7,100 samples.
In order to assure that the image brightness values of the orthophoto portray the source imagery as close as possible, very little image enhancement other than a limited amount of analog dodging is performed when preparing the photograph for scanning. Some deviation of brightness values may also occur during the scanning and rectification process. Radiometric accuracy and quality are accomplished through visual inspection of the digital orthophoto and comparison to the original unrectified image.
A standard USGS digital orthophoto is composed of the following elements:
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Earth Science Information Centers (ESICs)DOQ data are available on CD-ROMs and magnetic tape media. The DOQ data are in a JPEG compressed format on the CD-ROMs.
Applications and Related Data Sets
These digital cartographic and geographic data may be combined with other geographically referenced data, thus, enabling users to conduct automated analyses in support of various decision making processes. DOQ data may also be used as one layer in a geographic information system (GIS), as a tool for various kinds of spatial analyses, and as information for plotting base maps.
DOQs offer application possibilities that vectorized data do not. Much like the symbology on a topographic map, vector data tends to be generalized with only selected, cartographically significant information shown. A DOQ can offer a complete picture of all the distinguishable ground features. It may be cheaper and more efficient to produce accurate raster images of an area than it is to provide vectorized data of the same area. However, DOQs are not meant to be replacements for vector data.
The DOQ offers new levels of measurements and alternative viewing dimensions. For instance, when a DOQ is merged with a digital elevation model, three-dimensional modeling becomes possible.
Digital orthophoto quarter-quadrangles are based upon the North American Datum of 1983, in the Universal Transverse Mercator (UTM) projection, with coordinates in meters. The digital orthophoto quadrangles are cast on either NAD 27 or NAD 83, UTM projection, with coordinates in meters. The principal horizontal primary datum for the digital orthophoto quarter-quadrangle is NAD 83. The principal secondary horizontal datum for quarter-quadrangle orthophotos is the North American Datum of 1927 (NAD 27), the Puerto Rico Datum, the Old Hawaiian Datum, or other approved datums. The four primary datum corners are imprinted into the image as four solid white crosses (dn = 255) and the four secondary datum corners as four dashed white crosses (dn = 255).
Digital orthophoto accuracy is dependent upon it's input data meeting certain accuracy standards and the resultant digital orthophoto passing geometric quality assurance tests before acceptance into the USGS National Digital Cartographic Data Base (NDCDB). Almost all input necessary for constructing a digital orthophoto influence its accuracy and are subject to National Map Accuracy Standards (NMAS). The source aerial photography needs to meet NAPP specifications sufficient to meet NMAS. Since the vertical accuracy of the DEM can affect the horizontal accuracy of the DOQ, only source DEMs that are equivalent to or better than a level 1 DEM and have a root-mean-square-error (RMSE) of no greater than 7.0 meters are used in the production of digital orthophotos. Although digital orthophoto quadrangles and quarter-quadrangles are scaleless, they must meet horizontal accuracy requirements at 1:24,000 and 1:12,000 scale, respectively. The NMAS specify that 90 percent of the well-defined points tested must fall within 40 feet (1/50 inch) at 1:24,000 scale and 33.3 feet (1/30 inch) at 1:12,000 scale.
Digital orthophoto accuracy is expressed as RMSE and is determined by finding the line and sample coordinates in the digital orthophoto and comparing these to their ground coordinates. From four to nine points are checked. As a further accuracy test, the image line and sample coordinates of the DEM corners are transformed and compared with the actual X-Y DEM corner values to determine if they are within the RMSE.
Band Types and Order of Storage
Although the default band storage for DOQs is black and white, provisions have been made to accept other possible band types including red, green, and blue, with or without elevations; or combinations of those elements. The order of color band storage will always be red, green, blue, black and white, followed by elevations or some subset of those elevations. Elevation data can be added as profiles or pixels. When stored as profiles, pixel elevations may be determined by bilinear interpolation using the four adjacent profile elevations. The standard DOQ does not include elevation data; but if the option to include elevations is exercised, the data will be stored as a two-byte value.
Related to band type ordering is the format for storage patterns of the DOQ. Currently, the only option available is to store the monochrome image file and the DEM as separate files. Provisions have been made for both vertical storage (separate files for each band, or bands interleaved by line, record, or band), or horizontal storage (appending all bands for a given line or record into one line or record or interleaving bands by pixel for a given record).
The formats in which DOQ data may be stored include Band Interleaved by Line (BIL), Band Interleaved by Band (BSQ), Band Interleaved by Pixel (BIP), and as separate files.
An uncompressed DOQ file is organized into a series of four header records, each allocated as 400 bytes of ASCII text, followed by a series of 8-bit binary image data records. Each header record, from byte 401 - M, is padded with ASCII blanks to equal the length of the image data records. The minimum record length of 400 bytes is arbitrary but corresponds to the minimum practical length of image data records anticipated for any orthophoto file.
A wide range of descriptive information about the data image is included in the header records. File identification, coordinate systems and datum information, lineage and data source information relating to the input DEM, source photography and ground control information can be found in the header records.
Record 1
Record 1 contains information identifying the area of geographic coverage of the image including, quadrangle name and quadrant; Federal Information Processing Standard (FIPS) state and county codes for up to four States and five counties; information about data types and organization; primary and secondary datums information; ground coordinate data; and the location of the corner points in coordinates of the primary datum.
Record 2
Record 2 contains the transformation parameters and coordinates needed in order to convert from internal line and sample coordinates to the primary datum coordinate system. Also included are the ground coordinates of the quadrangle corners in the secondary horizontal datum.
Record 3
Record 3 contains the transformation parameters and coordinates to allow conversion from the internal line and sample coordinates to the secondary horizontal datum coordinate system. The line and sample coordinates of the corner points in the primary and secondary datums are found here, as are the first pixel (1,1) horizontal coordinates in both primary and secondary horizontal datums.
Record 4
Record 4 contains information related to the source DEM and aerial photography, processing systems, algorithms and software, and image quality.
Examples of Data Header Records
Notes on the North American Datum (NAD)
Unlike local surveys, which can treat the Earth as a plane, the precise determination of the latitude and longitude of points over a broad area must take into account the actual shape of the Earth. In order to achieve the precision necessary for very accurate location, the Earth cannot simply be assumed to be a sphere. Rather, the Earth's shape more closely approximates an oblate spheroid: flattened at the poles and bulging at the Equator. Thus, the Earth's shape, when cut along its polar axis, approximates an ellipse.
Geodetic surveying, which takes into account variations in the shape of the Earth, is based on a reference ellipsoid that is selected as a best fit to the actual shape of the Earth over a limited area. The Clarke 1866 ellipsoid was used for the North American Datum of 1927 (NAD 27). NAD 27, based on an initial point at Meades Ranch, Kansas, uses the parameters of the Clarke 1866 ellipsoid for the computation of the position of other points in the horizontal control network throughout North America. The ellipsoid used to define a datum is a mathematical surface upon which computations can be based, as opposed to the actual surface of the Earth on which surveys are conducted or the sea level surface to which they are reduced. The latter surface, the geoid, is an equipotential surface of the Earth's gravity field. It can be thought of as a continuous sea-level surface extended beneath the continents. It is the "level" surface of reference for astronomic observations and geodetic leveling, but because of undulations that respond to the Earth's mass distributions, it is not a useful computational surface for horizontal surveys.
Using modern geodetic, gravimetric, astrodynamic, and astronomic data, the Geodetic Reference System 1980 (GRS 80) ellipsoid, has been defined as a best fit to the world-wide geoid. Using GRS 80, the National Ocean Service has readjusted the North American Datum to produce the North American Datum of 1983 (NAD83). NAD 83 is an Earth-centered datum based on GRS 80. Because the NAD 83 surface deviates from the NAD 27 surface, the computed position of points determined using the two reference shapes will be different. In addition to changing the latitude and longitude values of map features, redefinition of the horizontal datum affects coordinates in the Universal Transverse Mercator (UTM) grid system. Because of the change in the ellipsoid, the UTM grid is shifted with respect to latitude and longitude coordinates. Thus, UTM coordinate differences between NAD 27 and NAD 83 will generally be much larger than the shifts in latitude and longitude.
The magnitude of shifts in UTM Eastings will be similar to the magnitude of shifts in longitude, however; shifts in UTM Northings will be about 200 meters larger than shifts in latitude. Examples of UTM coordinates in both NAD 27 and NAD 83 are shown below for three cities in the continental United States. Note the use of the lower-case letters d, m, and s for degrees, minutes, and seconds, respectively, in the geographic coordinate system.
Coordinate NAD27 NAD83 Difference in
system meters and seconds
Portland, OR
------------------------------------------------------------
| Geographic| | | |
| Latitude | 45d32m00.000N | 45d31m59.427N | -17.697m |
| | | | |
| | | | (-0.573s) |
| Longitude| 122d37m00.000W |122d37m04.341W | 94.185m |
| | | | |
| | | | ( 4.341s) |
|UTM | | | |
| Northing | 5,042,053.576m |5,042,253.069m | 199.493m |
| Easting | 529,931.820m | 529,836.863m | -94.957m **|
|-----------|----------------|---------------|-------------|
Kansas City, MO
------------------------------------------------------------
| Geographic| | | |
| Latitude | 39d06m00.000N | 39d06m00.016N | 0.487m |
| | | | |
| | | | ( 0.016s) |
| Longitude| 94d35m00.000W | 94d35m00.841W | 20.206m |
| | | | |
| | | | ( 0.841s) |
|UTM | | | |
| Northing | 4,328,858.897m |4,329,067.886m | 208.989m |
| Easting | 363,082.466m | 363,065.793m | -16.673m **|
|-----------|----------------|---------------|-------------|
Miami, FL
------------------------------------------------------------
| Geographic| | | |
| Latitude | 25d47m00.000N | 25d47m01.355N | 41.711m |
| | | | |
| | | | ( 1.355s) |
| Longitude| 80d11m00.000W | 80d10m59.170W | -23.113m |
| | | | |
| | | | ( 0.830s) |
|UTM | | | |
| Northing | 2,851,781.976m |2,851,985.411m | 203.435m |
| Easting | 581,882.640m | 581,904.019m | 21.380m **|
|-----------|----------------|---------------|-------------|
** Note: The algebraic sign of the Easting shift is
opposite that of the longitude shift because:
(1) west longitude is treated as positive here,
and (2) UTM False Eastings keep all UTM
coordinates positive.
m equals meters.
The following example illustrates the standard band storage option available at this time (band type code = 1, black and white, elevations not stored). The size of a given digital orthophoto may vary with the amount of overedge and the specific coordinates of the quadrangle. This example assumes a quarter-quadrangle digital orthophoto conforming to a 3.75-minute DEM with 30-meter grid spacing having 176 profiles and 226 elevations per profile is illustrated. A uniform overedge of 14 DEM grid posts (14 x 30 = 420 meters) on all sides is also assumed.
The digital orthophoto is from 1:40,000-scale NAPP photography scanned with a 25-micron spotsize and, therefore, has a one meter pixel resolution at ground scale. The size of the digital orthoph