Illinois Digital Orthophoto Quadrangles in UTM Zones 15 and 16, NAD83 - Collection of Quarter-Quadrangle Tiles

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Frequently-anticipated questions:

What does this data set describe?

Illinois Digital Orthophoto Quadrangles in UTM Zones 15 and 16, NAD83 - Collection of Quarter-Quadrangle Tiles
This is a collection of tile-by-tile NAPP-III DOQs (1998-2001) images in MrSID format for the state of Illinois, in UTM Zones 15 and 16, NAD83.

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.

  1. How should this data set be cited?

    U.S. Geological Survey, 2000, Illinois Digital Orthophoto Quadrangles in UTM Zones 15 and 16, NAD83 - Collection of Quarter-Quadrangle Tiles:, Illinois State Geological Survey, Champaign, IL.

    Online Links:

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -91.5336
    East_Bounding_Coordinate: -87.2490
    North_Bounding_Coordinate: 42.6108
    South_Bounding_Coordinate: 36.8568

  3. What does it look like?

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 1998
    Ending_Date: 2001
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: remote-sensing image

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a raster data set. It contains the following raster data types:

      • Dimensions varies x varies, type Pixel

    2. What coordinate system is used to represent geographic features?

      Grid_Coordinate_System_Name: Universal Transverse Mercator
      UTM_Zone_Number: 15 or 16
      Scale_Factor_at_Central_Meridian: .09996
      Longitude_of_Central_Meridian: -93 (or -87 for zone 16)
      Latitude_of_Projection_Origin: 0.0
      False_Easting: 500000.
      False_Northing: 0.0

      Planar coordinates are encoded using row and column
      Abscissae (x-coordinates) are specified to the nearest 1
      Ordinates (y-coordinates) are specified to the nearest 1
      Planar coordinates are specified in meters

      The horizontal datum used is North American Datum 1983.
      The ellipsoid used is Geodetic Reference System 80.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257.

  7. How does the data set describe geographic features?

    For DOQ's from panchromatic source each pixel contains an 8-bit gray-scale value between 0-255. A value of 0 represents the color black while a value of 255 represents the color white. All values between 0 and 255 are represented as a shade of gray varying from black to white. Areas where the rectification process is incomplete due to incomplete data (i.e., lack of elevation data, gaps), are represented with the numeric value of 0.
    U.S. Department of the Interior, U.S. Geological Survey, 1992, Standards for digital orthophotos: Reston, VA. A hypertext version is available at: <URL:>

    Softcopy in ASCII format is available at: <URL:> <URL:>

    Softcopy in WordPerfect format is available at: <URL:> <URL:>

    Softcopy in PostScript format is available at: <URL:> <URL:>

    Softcopy in hypertext is available at: <URL:>

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

    Metadata written by Chris McGarry, after USGS metadata.

  3. To whom should users address questions about the data?

    Contact_Instructions: Please refer to the Distribution Information section.

Why was the data set created?

DOQ's serve a variety of purposes, from interim maps to field references for earth science investigations and analysis. The DOQ is useful as a layer of a geographic information system and as a tool for revision of digital line graphs and topographic maps.

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.

How was the data set created?

  1. From what previous works were the data drawn?

    DEM1 (source 1 of 4)
    U.S. Geological Survey, unknown, digital elevation model: U.S. Geological Survey, Reston, VA.

    Type_of_Source_Media: CD
    Elevation data in the form of an ortho-DEM regridded to user-specified intervals and bounds.

    PHOTO1 (source 2 of 4)
    U.S. Geological Survey, unknown, Aerial Photography (photo ID number): U.S. Geological Survey, Reston, VA.

    Type_of_Source_Media: cartridge tape
    Source_Scale_Denominator: 40000
    Panchromatic black and white NAPP photograph. NAPP photographs are centered on the DOQ coverage area.

    CONTROL_INPUT (source 3 of 4)
    U.S. Geological Survey, Unpublished material, project ground and photo control: U.S. Geological Survey, Reston, VA.

    Type_of_Source_Media: various media
    Horizontal and vertical controls used to establish positions and elevations for reference and correlation purposes.

    CAMERA_INPUT (source 4 of 4)
    U.S. Geological Survey, Unpublished material, report of calibration: U.S. Geological Survey, Reston, VA.

    Type_of_Source_Media: disk, paper
    Source_Contribution: Camera calibration parameters

  2. How were the data generated, processed, and modified?

    Date: 2000 (process 1 of 1)
    The production procedures, instrumentation, hardware andsoftware used in the collection of standard USGS DOQ'svary depending on systems used at the contract, cooperatoror USGS production sites. The majority of DOQ datasetsare acquired through government contract. The process stepdescribes, in general, the process used in the productionof standard USGS DOQ data sets. Additional detail isincluded for the modifications made to the original,standard USGS DOQs for enhancement and online distributionthrough the ISGS.

    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

    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.

    Data sources used in this process:


  3. What similar or related data should the user be aware of?

    , ISGS Terms of Use.

    Online Links:

    , University of Illinois Web Privacy Notice.

    Online Links:

    , University Copyright Policy (stated in the General Rules Concerning University Organization and Procedure, Article III, Section 4).

    Online Links:

    , Policy on Appropriate Use of Computers and Network Systems at the University of Illinois at Urbana-Champaign.

    Online Links:

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

    For original, uncompressed DOQs (GeoTIFF format): During photographic reproduction of the source photography, limited analog dodging is performed to improve image quality. Analog dodging consists of holding back light from certain areas of the sensitized photographic material to avoid overexposure. The diapositive is inspected to insure clarity and radiometric uniformity. Diapositive image brightness values are collected with a minimum of image quality manipulation. Image brightness values may deviate from brightness values of the original imagery due to image value interpolation during the scanning and rectification processes. Radiometry is verified by visual inspection of the digital orthophoto quadrangle with the original unrectified image to determine if the digital orthophoto has the same or better image quality as the original unrectified input image. Slight systematic radiometric differences can be detected between adjacent DOQ files due primarily to differences in source photography capture dates and sun angles of aerial photography along flight lines. These differences can be observed in an image's general lightness or darkness when compared to adjacent DOQ file coverages.

    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.

  2. How accurate are the geographic locations?

    The DOQ horizontal positional accuracy and the assurance of that accuracy depend, in part, on the accuracy of the data inputs to the rectification process. These inputs consist of the digital elevation model (DEM), aerotriangulation control and methods, the photo source camera calibration, scanner calibration, and aerial photographs that meet National Aerial Photography Program (NAPP) standards. The vertical accuracy of the verified USGS format DEM is equivalent to or better than a USGS level 1 or 2 DEM, with a root mean square error (RMSE) of no greater than 7.0 meters. Field control is acquired by third order class 1 or better survey methods sufficiently spaced to meet National Map Accuracy Standards (NMAS) for 1:12,000-scale products. Aerial cameras have current certification from the USGS, National Mapping Division, Optical Science Laboratory. Test calibration scans are performed on all source photography scanners. Horizontal positional accuracy is determined by the orthophoto Accuracy (ORACC) software program for DOQ data produced by the National Mapping Division. The program determines the accuracy by finding the line and sample coordinates of the passpoints in the DOQ and fitting these to their ground coordinates to develop a root mean square error (RMSE). From 4 to 9 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. Additional information on this testing procedure can be found in U.S. Department of the Interior, U.S. Geological Survey, 1993, Technical Instructions, ORACC Users Manual (draft): Reston, VA. Adjacent DOQ's, when displayed together in a common planimetric coordinate system, may exhibit slight positional discrepancies across common DOQ boundaries. Linear features, such as streets, may not be continuous. These edge mismatches, however, still conform to positional horizontal accuracy within the NMAS. Field investigations to validate DOQ positional accuracy reliability are periodically conducted by the USGS, National Mapping Division, Geometronics Standards Section. DOQ's produced by cooperators and contractors use similarly approved RMSE test procedures.

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    All DOQ imagery is visually inspected for completeness to ensure that no gaps, or image misplacement exist in the 3.75' image area or in overedge coverage. DOQ images may be derived by mosaicking multiple images, in order to insure complete coverage. All DOQ's are cloud free within the 3.75' image area. Some clouds may, very infrequently, be encountered only in the overedge coverage. Source photography is leaf-off in deciduous vegetation regions. Void areas having a radiometric value of zero and appearing black may exist. These are areas for which no photographic source is available or result from image transformation from other planimetric systems to the Universal Transverse Mercator (UTM). In the latter case, the void sliver areas are on the outside edges of the overedge area. The data set field content of each DOQ header record element is validated to assure completeness prior to archiving in the NDCDB.

  5. How consistent are the relationships among the observations, including topology?

    All DOQ header data and image file sizes are validated by the Tape Validation System (TVS) software prior to archiving in the National Digital Cartographic Data Base (NDCDB). This validation procedure assures correct physical format and field values for header record elements. Logical relationships between header record elements are tested. Header data for each DOQ has been extracted and is available in the doqindex.dbf file. An explanation of each of the headings in doqindex.dbf file can be found in READMEDOQ.txt.

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

ISGS information must be obtained directly from the ISGS or from an authorized distributor. Be aware that ISGS information obtained from an unauthorized third party may have been altered subsequent to original distribution, or may no longer be current.

Any access to these data, Web sites, computer systems, electronic services, and electronic networks is governed by University and campus policies, in particular, but not limited to, the Policy on Appropriate Use of Computers and Network Systems at the University of Illinois at Urbana-Champaign, the ISGS Terms of Use document available at the ISGS Web site, and the University of Illinois Web Privacy Notice.

Links to these are provided in the Cross References section.

ISGS information is the property of and copyrighted by the Board of Trustees of the University of Illinois with all rights reserved.  University copyright policy is stated in the General Rules Concerning University Organization and Procedure, Article III, Section 4. A link is 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, License fees and a license agreement may be required, depending on the proposed usage.

Any use of these data is governed by University and campus policies, in particular, but not limited to, the Policy on Appropriate Use of Computers and Network Systems at the University of Illinois at Urbana-Champaign, the ISGS Terms of Use document available at the ISGS Web site, and the University of Illinois Web Privacy Notice. Links to these are provided in the Cross References section.

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.

  1. Who distributes the data set? (Distributor 1 of 1)

    Information Office, Illinois State Geological Survey (ISGS), a division of the Institute of Natural Resource Sustainability (INRS) of the University of Illinois
    615 East Peabody Drive
    Champaign, Illinois 61820

    217-333-4747 (voice)

    Hours_of_Service: 8:00 a.m. - 4:30 p.m., Monday-Friday, U.S. Central Time
  2. What's the catalog number I need to order this data set?

  3. What legal disclaimers am I supposed to read?

    Refer to the Legal Notices of Terms and Conditions of the University of Illinois Web Privacy Policy (there is a link in the Cross References section) for policy statements regarding the following:

    Disclaimer of Liability Disclaimer of Warranties and Accuracy of Data Disclaimer of Endorsement Disclaimer for External Links Disclaimer of Duty to Continue Provision of Data Security Choice of Law

    By obtaining these data you agree to the provisions of the University of Illinois Web Privacy Policy, regardless of the manner in which the information was obtained.

  4. How can I download or order the data?

  5. What hardware or software do I need in order to use the data set?

    The data are intended for use with GIS software. The ISGS uses ESRI ArcGIS software, however, ESRI formats can be imported into many different GIS software packages. It is expected that customers who obtain these data have the technical expertise to use GIS software. The ISGS does not provide software support of any kind.

Who wrote the metadata?

Last modified: 24-Feb-2009
Metadata author:

Contact_Instructions: Please refer to the Distribution Information section.
Metadata standard:
FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:

Illinois Natural Resources Geospatial Data Clearinghouse

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