The National Imagery and Mapping Agency (NIMA) (formerly the Defense Mapping Agency) adopted a special grid for military use throughout the world called the Universal Transverse Mercator (UTM) grid. In this grid, the world is divided into 60 north-south zones, each covering a strip 6° wide in longitude. These zones are numbered consecutively beginning with Zone 1, between 180° and 174° west longitude, and progressing eastward to Zone 60, between 174° and 180° east longitude. Thus, the conterminous 48 States are covered by 10 zones, from Zone 10 on the west coast through Zone 19 in New England (Fig. 1). In each zone, coordinates are measured north and east in meters. (One meter equals 39.37 inches, or slightly more than 1 yard.) The northing values are measured continuously from zero at the Equator, in a northerly direction. To avoid negative numbers for locations south of the Equator, NIMA's cartographers assigned the Equator an arbitrary false northing value of 10,000,000 meters. A central meridian through the middle of each 6° zone is assigned an easting value of 500,000 meters. Grid values to the west of this central meridian are less than 500,000; to the east, more than 500,000. (USGS Fact Sheet 077-01, August 2001) All the DRGs in the NSDD are in the UTM NAD 83 Zone 15 coordinate system. The NSDD DRG images can be reprojected to any coordinate system using the free ERDAS ViewFinder software listed below under tools.

Data File Types

All NSDD DOQs are in Zip (*.zip) format. The name of the files are the same as the USGS map sheet. If you do not have a zip utility, you can follow this link to download one Here. The following information will give you more detail on the DOQs and their associate files.

The NSDD DOQs are in either the Tagged Image File Format (TIFF) or the  Multi-resolution Seamless Image Database (MrSID) file format. MrSID is a high-quality, high-performance wavelet based lossy compression methodology designed specifically to enable true portability of massive images. The MrSID Encoder is developed and supported by LizardTech, Inc.  Many remote sensing and Geographic Information System (GIS) support the MrSID file format.  If they don't support the MrSID format, you can use a free software package (MrSIDViewer) to export the MrSID file, or a subset of the image file, into the TIFF file format.

World File (Coordinate Information)

All the NSDD DOQs have World files (*.tfw for TIFF files and *.sdw for MrSID files). They have been included as a service to users of ESRI software and other commercial software that uses ESRI's world files. The world file contains the ground coordinates of the upper left pixel of the TIFF image (the (1,1) or (0,0) pixel, depending on convention), and scale and rotation information. World files do not contain projection and datum information. More information about world files can be found at http://support.esri.com.

AUX File (Projection and Datum Information)

An Auxiliary file (.aux) is a file that accompanies the raster in the same location and stores any auxiliary information that cannot be stored in the raster file itself. This is where statistical information for the raster dataset will be stored, if it cannot be stored internally. Color map, histogram/table, coordinate system, transformation, and projection information may also be stored in the AUX file.  The information stored in an AUX file is only accessible using a product from ESRI^®, ERDAS or a third-party product derived from their products.  All the NSDD DOQs have an *.aux file association with them that stores the projection information of the DOQ.

RRD File (Pyramid Layer)

The Pyramid layer (*.rrd) is a file that we have created for all tiff DOQs in the NSDD.  This file speeds up the  drawing of a tiff when using a product from ESRI^®, ERDAS or a third-party product derived from their products. The amount of detail you see when drawing a raster data layer (DRGs, scanned photograph, etc.) depends on its cell or pixel size. If each cell covers a very small area so that details are maintained, then more cells are required to cover the same area; more detailed raster data require more space on disk and also takes longer to draw. To speed up the display of large raster images, we created pyramid layers, in which the original data has several levels of resolution. With pyramids, a lower resolution copy of the data displays quickly when drawing the entire dataset. As you zoom in, levels with finer resolutions are drawn; performance is maintained because you're drawing successively smaller areas. Without pyramids, the entire raster dataset must be queried to determine the subset of cells that need to be displayed.  Pyramid layers provide a significant improvement in drawing performance.

Tools:

Forestry GIS (fGIS)

Forestry GIS (fGIS™) is a Freeware Mapping Utility for Resource Managers and a compact shapefile editing program, digitizer and GIS data query tool for Windows®. Designed for operational field managers like foresters and wildlife biologists, fGIS has essential tools tailored for precision mapping on a desktop computer. Use fGIS to customize layered views including aerial photos and other imagery, draw map objects, query and search spatial data, annotate maps for printing, and much more. The standalone program is unlimited, producing shapefiles and geo-referenced images compatible with commercial software in an enterprise GIS framework. Forestry GIS is meant to be part of a hierarchy of GIS tools, with ArcView/ArcGIS being the WI DNR standard for advanced GIS users.

Forestry GIS (fGIS™) can be downloaded from the following link: http://www.digitalgrove.net/fgis.htm. Updates to fGIS are posted regularly at this link, so it's a good idea to check back often.
You'll find that fGIS is relatively simple to use:

* Open and view geo-referenced raster images including digital orthophotos and topographic maps in MrSID®, ECW, BIL, JPG, TIFF, GeoTIFF, BMP and PNG file formats.

* Open and view vector data in ESRI® shapefile (SHP) and e00 files, AutoCAD DXF™, MapInfo® MIF, Microstation® DGN, DLG-O, TIGER2000/line and TatukGIS SQL (ttkls) file formats. An advanced R-Tree index system allows easy viewing of large (100+ MB) data files.

* Create and edit point, line and area shapefile objects and their associated data attribute tables.

* Designate the direction and distance of the next leg of a line or polygon with the COGO-like Traverse Tool. Directions can be entered in decimal degrees or degrees-minutes-seconds (handy for plotting deeds or laying out a field data collection route).

* Split a polygon into smaller units by drawing a line with the Split Shapes Tool. Area and perimeter attributes of forest cover type subunits are automatically updated.

* Customize map properties including line width and color, area fills, label fonts and positions, image transparency, thematic rendering of data and more.

* Measure areas and distances. fGIS reports distances and areas in English units (feet, miles and acres) and metric units.

* Display data at user-specified scales.

* Extract geo-referenced images of selected portions of a screen display.

* Export part of a vector layer and save it as a new file.

* Manage large data sets organized in dBase tables with the "Load Library Layer" utility.

* Join dBase and Access tables to shapefile attribute tables.

* Create point shapefiles from comma delimited text files with x|y coordinates.

* Print the map with three flexible output options. The Simple Print command will add a map title, subtitle, footer and the scale to the page. The Send Map to Word command will send an image of the view at a user specified resolution and scale to Microsoft® Word. A third option through the Export Image tool will send the map to a technical illustration/page layout module called Diagram Designer. You can add headers, legends, annotations and non-spatial symbols from template palettes in the Diagram Designer module.

Zip Utility

All NSDD DOQs are in .zip format. If you do not have a zip utility, you can download the file.

MrSIDViewer

The free MrSID Viewer is licensed from International Land Systems, Inc. and is a cooperative product with Lizardtech, Inc. ILS can be reached over the Internet at http://www.landsystems.com. The MrSID Viewer displays images that have been compressed into the MrSID format.  Currently, the MrSID Viewer is available for Windows 95/NT, IRIX 6.2, Linux 2.0.32 and Solaris 2.5.1.  The MrSID Viewer can be download from Lizardtech, Inc. (Version: 2.0.0.50).   The SAL has created a lesson (PDF format) that explains how to subset a MrSID image and export it as a TIFF Image file.    The lesson requires Adobe Reader.

ERDAS ViewFinder 2.1

ERDAS ViewFinder is a free viewing tool provided by Leica Geosystems' GIS & Mapping Division.  It provides basic image viewing and manipulation capabilities, including the ability to: 

The ERDAS ViewFinder can be download from ERDAS.  

MapSheets Express v1.3

MapSheets Express is the FREE viewing and mapping tool for working with imagery and vector map data.

Mapsheets Express v1.3 can be download from ERDAS.  

Acquisition

Digital orthophotos require several types of inputs to produce an orthogonally rectified image from the original perspective image captured by the sensor.

These inputs are the following:

1) the unrectified raster image scanned from the diapositive or directly acquired from a digital sensor,
2) a digital elevation model with the same area of coverage as the digital orthophoto,
3) the image and ground coordinates of photo identifiable ground control points,
4) calibration information about the sensor collector device and,
5) a user parameter file.

These five inputs are used to register the image file to the scanner and to the sensor platform, to determine the orientation and location of the sensor platform with respect to the ground, and to remove the relief displacement from the image data.

Data Characteristics

Resolution is the minimum distance between two adjacent features or the minimum size of a feature, that can be detected by a remote sensing system. The resolution is generally larger than the computed ground sample distance of the DOQ. The ground sample distance (GSD) is the distance on the ground represented by each pixel in the x and y components. The ground sample distance of the digital orthophoto is a result of the scanning aperture of the microdensitometer used to capture the digital image and the resampling algorithm.

For example, if a scanning aperture of 25 micrometers is used on a 1:40,000 photo-scale image, the ground (pixel) sample distance is approximately 1 meter. A 7.5 micrometer scan yields a pixel size of 0.3 meters while a 15 micrometer scan equates to a 0.6 meter. For the processed DOQ, the GSD is 1 meter for quarter-quad digital orthophoto and 2 meters for quadrangle digital orthophotos. If digital orthophotos are produced at a finer sampling distance than 1 or 2 meters, they may be processed by resampling to 1 or 2 meter horizontal GSD. Digital orthophotos produced at a coarser sample distances are not resampled to a finer horizontal ground sample distance.

The geographic extent of the digital orthophoto is equivalent to an orthophoto quarter-quadrangle (3.75-minutes of latitude and longitude), plus a minimum of 50 meters to to a maximum of 300 meters of overedge is included, sufficient to offer 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 lines (rows) and samples (columns).

Spectral Range

In order to assure that the image brightness values of the orthophoto closely portray the source imagery, 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 processes. Radiometric accuracy and quality are verified through visually inspecting and comparing the digital orthophoto to the original unrectified image.

Accuracy

Digital orthophoto quadrangles and quarter-quadrangles must meet horizontal National Map Accuracy Standards (NMAS) at 1:24,000 scale 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. The vertical accuracy of the source DEM must be equivalent to or better than a level-1 DEM, with a root-mean-square-error (RMSE) of no greater than 7 meters. The DOQ RMSE is the square root of the average of the squared discrepancies.

These discrepancies are the differences in coordinate (x and y) values derived by comparing the data being tested with values determined during aerotriangulation or by an independent survey of higher accuracy. All remaining inputs and processes (e.g., aerotriangulation control and methodology, scanner and sensor calibrations) used in digital orthophoto production must be sufficiently accurate to ensure that the final product meets NMAS specifications.