Orthophotos combine the image characteristics of aphotograph with the geometric qualities of a map. Theprimary digital orthophotoquad (DOQ) is a 1-meter groundresolution, quarter-quadrangle (3.75-minutes of latitudeby 3.75-minutes of longitude) image cast on the UniversalTransverse Mercator Projection (UTM) on the North AmericanDatum of 1983 (NAD83).
The geographic extent ofthe DOQ is equivalent to a quarter-quad plus the overedge,which ranges a minimum of 50 meters to a maximum of 300 metersbeyond the extremes of the primary (NAD83) and secondary(NAD27) corner points. The overedge is included to facilitatetonal matching for mosaicking and for the placement of primaryand secondary datum corner ticks. The normal orientationof data is by lines (rows) and samples (columns). Eachline contains a series of pixels ordered from west toeast with the order of the lines from north to south.The standard, archived, USGS digital orthophoto is formattedas four ASCII header records, followed by a series of 8-bitbinary image data records. The radiometric imagebrightness values are stored as 256 gray levels rangingfrom 0 to 255. The modified, compressed ISGS digitalorthophoto is a black-and-white image in MrSID .sidformat. The target compression ratio of 15:1 maintainssufficient detail for most mapping projects.The metadata provided in the doqindex.dbftable contain a wide range of descriptiveinformation including format source information,production instrumentation and dates, and data to assistwith displaying and georeferencing the image.
These data are appropriate for use in local and regional thematic analysis. The data are not appropriate as a geodetic, legal or engineering base. It serves as an important base cartographic element for many types of maps and has been widely distributed and used outside the ISGS. The data set was not and is not intended as a substitute for surveyed locations, such as can be determined by a registered Public Land Surveyor. Although useful in a GIS as a reference base layer for maps, the data set has no legal basis in the definition of boundaries or property lines.
Links to these are provided in the Cross References section.
Individuals or entities may make fair use of copyrighted ISGS material, such as reproducing a single figure or table, or using a brief text quotation, without obtaining formal permission, but in all cases the Illinois State Geological Survey must be credited as the source of the material. To reproduce ISGS information beyond the fair use standard, permission must be obtained from the ISGS Information Office, 615 East Peabody Drive, Champaign, Illinois 61820, 217-333-4747, email@example.com. License fees and a license agreement may be required, depending on the proposed usage.
Map information is to be used at a scientifically and cartographically appropriate scale, that is, at a scale no greater than indicated on the map or as described in the documentation of the map or map data. Map information is not appropriate for, and is not to be used as, a geodetic, legal, or engineering base. Map information has no legal basis in the definition of boundaries or property lines and is not intended as a substitute for surveyed locations such as can be determined by a registered Public Land Surveyor.
The data do not replace the need for detailed site-specific studies.
For modified, compressed DOQs (MrSID format): Image brightness and contrast has been enhanced for a more aesthetic and useful look since the original imagery was rather dark. This adjustment was conducted in image processing software, PCI. The resultant, modified image was compressed using MrSID at a target compression ratio of 15:1. MrSID is not lossless compression, however image degradation is only noticeable with extremely fine detail (i.e. plowing patterns in agricultural fields) when zoomed in to a level where the original imagery begins to pixelate.
The rectification process requires, as input, a userparameter file to control the rectification process, adigital elevation model (DEM1) gridded to user specifiedbounds, projection, zone, datum and X-Y units, a scanneddigital image file (PHOTO1) covering the same area as theDEM, ground X-Y-Z point values (CONTROL_INPUT) and theirconjugate photo coordinates in the camera coordinate system,and measurements of the fiducial marks (CAMERA_INPUT) inthe digitized image.
The camera calibration report (CAMERA_INPUT) provides thefocal length of the camera and the distances in millimetersfrom the camera's optical center to the camera's 8 fiducialmarks. These marks define the frame of reference for spatialmeasurements made from the photograph. Ground controlpoints (CONTROL_INPUT) acquired from ground surveys ordeveloped in aerotriangulation, are third order class 1 orbetter, and meet National Map Accuracy Standard (NMAS) for1:12,000-scale. Ground control points are in the UniversalTransverse Mercator or the State Plane Coordinate System onNAD83. Horizontal and vertical residuals of aerotriangulatedtie-points are equal to or less than 2.5 meters. Standardaerotriangulation passpoint configuration consists of 9ground control points, one near each corner, one at thecenter near each side and 1 near the center of thephotograph, are used. The conjugate positions of the groundcontrol points on the photograph are measured and recordedin camera coordinates.
The raster image file (PHOTO_1) is created by scanning anaerial photograph film diapositive with a precision imagescanner. An aperture of approximately 25 to 32 microns isused, with an aperture no greater than 32 microns permitted.Using 1:40,000-scale photographs, a 25-micron scan apertureequates to a ground resolution of 1-meter. The scannerconverts the photographic image densities to gray scalevalues ranging from 0 to 255 for black and white photographs.Scan files with ground resolution less than 1 meter orgreater than 1 meter but less than 1.28 meters are resampledto 1 meter.
The principal elevation data source (DEM1) are standardDEM datasets from the National Digital Cartographic DataBase (NDCDB). DEM's that meet USGS standards are alsoproduced by contractors to fulfill DOQ production requirementsand are subsequently archived in the NDCDB. All DEM data isequivalent to or better than USGS DEM standard level 1. TheDEM used in the production of DOQ's generally has a 30-metergrid post spacing and possesses a vertical RMSE of 7-metersor less. A DEM covering the extent of the photograph is usedfor the rectification. The DEM is traversed from user-selected minimum to maximum X-Y values and centered on theDOQ coverage area. The DEM X-Y-Z values are used to findpixel coordinates in the digitized photograph usingtransformations mentioned above. For each raster imagecell subdivision, a brightness or gray-scale value isobtained using nearest neighbor, bilinear, or cubicconvolution resampling of the scanned image. The pixelprocessing algorithm is indicated in the header file. Aninverse transformation relates the image coordinatesreferenced to the fiducial coordinate space back to scannercoordinate space. For those areas for which a 7.5-minuteDEM is unavailable and relief differences are less than150 feet, a planar-DEM (slope-plane substitute grid) maybe used.
Rectification Process:The photo control points and focal length areiteratively fitted to their conjugate groundcontrol points using a single photo space resectionequation. From this mathematical fit is obtained a rotationmatrix of constants about the three axes of the camera.This rotation matrix can then be used to find thephotograph or camera coordinates of any other groundX-Y-Z point. Next a two dimensional fit is made between themeasured fiducial marks on the digitized photograph and theirconjugate camera coordinates. Transformation constants aredeveloped from the fit and the camera or photo coordinatesare used in reverse to find their conjugate pixel coordinateson the digitized photograph.
Quality Control:All data is inspected according to a quality control plan.DOQ contractors must meet DOQ standards for attributeaccuracy, logical consistency, data completeness andhorizontal positional accuracy. During the initialproduction phase, all rectification inputs and DOQ datasets are inspected for conformance to standards. After aproduction source demonstrates high quality, inspectionswill be made to 10% of delivery lots (40 DOQs per lot). AllDOQ's are visually inspected for gross positional errors andtested for physical format standards.
Post-processing by Illinois State Geological Survey:
Upon receipt from the USGS, the DOQ imagery was initiallychecked for spatial integrity, image quality, and accurateassociated header metadata. Each DOQ was imported intoremote-sensing software, PCI, from PCI Geomatics, forbrightening and increased contrast through the NormalizationEnhancement function. The modified imagery was exported as aTIFF file with an accompanying TFW world file. The resultantTIFF file was converted to an ArcInfo GRID, which was thenprojected from the original UTM, NAD83 to Lambert ConformalConic, NAD27. The GRID was then exported back to a new TIFFwith an accompanying TFW world file. This TIFF file wascompressed using MrSID, image compression software fromLizardTech, Inc. A target compression ratio of 15:1, intensityweight of 4 and a gamma of 2 were used in the compression.The resultant imagery has a ".sid" extension and the associatedgeoreferenced world file has a ".swd" extension. MrSID minimizesimage file size while retaining usability. Although compressionresults in a very slight distortion from the original image,degradation is not noticeable at a compression ratio of 15:1.For more information on MrSID and the wavelet compression used,visit www.lizardtech.com.
Elements pertinent to individual DOQs were extractedfrom the DOQ header data and the MrSID output.These elements were placed in a dBASE file, doqindex.dbf.Additional metadata describing the elements in thedoqindex.dbf file can be found in READMEDOQ.txt.
DOQs were reprojected from the original UTM (zone 15/16),NAD83 to Lambert Conic Conformal projection using ESRI products.
Softcopy in ASCII format is available at: <URL:ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt1.txt> <URL:ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt2.txt>
Softcopy in WordPerfect format is available at: <URL:ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt1.wp5> <URL:ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt2.wp5>
Softcopy in PostScript format is available at: <URL:ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt1.ps> <URL:ftp://www-nmd.usgs.gov/pub/ti/DOQ/doqstnds/stdoqpt2.ps>
Softcopy in hypertext is available at: <URL:ftp://www-nmd.usgs.gov/pub/doq_html/standards_doq.html>
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