Basic Orthophoto and Mapping Glossary*

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What is a digital orthophoto?

Digital orthophotos are photographic images that combine the image characteristics of an aerial photograph with the geometric qualities of a map.  Ordinary aerial photographs have inconsistencies in scale due to variations in terrain elevation, changes in distance from the camera to the terrain across the field-of-view, and tilting of the camera itself when the plane is not flying perfectly straight and level.  To create an orthophoto, an aerial photograph is scanned using a precise, high-resolution scanner.  Each image pixel is then processed through photogrammetric equations using ground control points, camera calibration and orientation parameters, and a digital elevation model.  The result is an orthorectified image in which distortions and displacements are removed, allowing distances, areas, and angles to be precisely measured.

TeraServerUSA orthophotos have a ground resolution of 1 meter (i.e. each pixel represents 1 m2) at highest resolution and a nominal scale of  1:12,000.  The images use a Universal Transverse Mercator (UTM) grid based on the North American Datum of 1983 (NAD83).
 


What is a world coordinate file?

A world file is a small text file that accompanies some image file formats such as JPG, PNG, BMP and TIFF. The corresponding world file for such images would be given JGW, PGW, BPW or TFW file extensions. The first part of an image and corresponding world file must have the same name (such as mountain.jpg and mountain.jgw) and be located in the same folder.

The world file provides information that can be used to register an image to real-world coordinates.  It contains parameters in plain text which can be used to establish an image-to-world transformation that converts the image coordinates to real-world coordinates.  Some GIS programs will automatically use this information to register an image when you display it.  The contents of the world file might look something like this:

1.282090
0.000000
0.000000
-1.282090
730691.000000
4797771.000000

 

World file parameters are always stored in this order:

A. X-scale (meters per pixel in the X direction)

1.282090

D. Rotation in X direction (assumed = 0)

0.000000

B. Rotation in Y direction (assumed = 0)

0.000000

E. Negative of Y-scale (meters per pixel in the Y direction)

-1.282090

C. Easting Coordinate of the center of the upper left pixel of the image

730691.000000

F. Northing Coordinate of the center of the upper left pixel of the image

4797771.000000

Note that simple world files contain no information about the type of projection or coordinate system and datum used to calibrate the image. You'll need to supply that information (often found in metadata text files from distributors of geographic data) if your spatial mapping program requests it. Some programs like fGIS will not require a name for the projection or coordinate system and datum, but will require all data to be based on a similar system in order for layers to line up. FYI, GeoTiff images have headers that contain information similar to world files. GeoTiff headers may also have flags for additional information like projection, datum, spheroid, etc.

If you want to create a world file for an image, see the Digital Gove tutorial for using the TatukGIS Viewer and HyperCube (two freeware programs) for image calibration and rectification.

The world file values can be used in a six-parameter affine transformation in the form of:

  x1 = Ax + By + C
  y1 = Dx + Ey + F

where

  x1 = calculated x-coordinate of the pixel on the map
  y1 = calculated y-coordinate of the pixel on the map
   x = column number of a pixel in the image
   y = row number of a pixel in the image
   A = x-scale; dimension of a pixel in map units in x direction
 B,D = rotation terms (assumed to be zero)
 C,F = translation terms; x,y map coordinates of the center
       of the upper-left pixel
   E = negative of y-scale; dimension of a pixel in map units
       in y direction

Affine transformations involve rotating, scaling and skewing an image in a manner that maintains the relative position of points within the image (e.g., parallel lines stay parallel).

Note:  The y-scale (E) is negative because the origins of an image and a geographic coordinate system are different. The origin of an image is located in the upper-left corner, whereas the origin of the map coordinate system is located in the lower-left corner. Row values in the image increase from the origin downward, while y-coordinate values in the map increase from the origin upward.
 

What is a Map Projection?
The curved, three-dimensional surface of the earth is difficult to represent on a flat, two-dimensional map.  A map projection defines the spatial relationship between features on the earth's surface (3D) and their representations on a map (2D). It is a mathematical expression based on a sphere or spheroid, (conic, cylindrical, or planar) which transforms the earth's curved terrain to a flat surface.  Map projections cause distortion to one or more map properties, such as scale, shape, area, distance, or direction.  Hundreds of map projections have been developed to accurately represent at least one map property.   A map projection is selected based on scale and which map property must be preserved.

What are Coordinate or Grid Systems?
A coordinate system is a reference grid used to locate the position of features on a map.  It is a two-dimensional grid displayed as rows and columns with each presenting a unit of distance. Out of convenience, UTM and Geographic (Lat/Lon) systems are often referred to as map "projections". Technically, however, they are coordinate systems rather than projections as defined above.

  • The Universal Transverse Mercator (UTM) coordinate system divides the globe into sixty zones, each spanning six degrees of longitude. Each zone has its own central meridian from which it spans 3 degrees west and 3 degrees east. See the Colorado State University UTM page for a complete explanation.
  • Geographic (Latitude - Longitude) is a coordinate system that treats the globe as a sphere divided into 360 equal parts called degrees. Each degree can be further subdivided into 60 minutes, each composed of 60 seconds. The standard origin is where the Greenwich Prime Meridian meets the Equator. All points north of the Equator and east of the Prime Meridian are positive. The origin divides the globe into four quadrants; northwest, northeast, southwest and southeast. Each line of longitude runs north and south and measures the number of degrees east or west of the Prime Meridian. Values range from positive 180(eastern hemisphere) to negative 180 degrees (western hemisphere). Lines of latitude are parallel to the Equator and run from east to west. They measure the number of degrees north or south of Equator. (A popular saying is "changes in latitude bring changes in attitude.") Values range from +90 at the North Pole to -90 degrees at the South Pole.  

What is a Datum? 
The earth's surface is not perfectly round, but shaped as an ellipsoid.  Datums were developed to accurately map topographic differences in the earth's surface based on an ellipsoid.  A datum is a set of parameters defining a coordinate system, and a set of control points whose geometric relationships are known, either through measurement or calculation (Dewhurst 1990).

Most of the spatial data available from the USGS use one of the following two horizontal datums used almost exclusively in North America, the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83).

    North American Datum of 1927 (NAD27)
    The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent the shape of the earth. The origin of this datum is a point on the Earth referred to as Meades Ranch in Kansas. Many NAD27 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years. Therefore, errors varied from station to station. There may be as much as 300 feet difference in positions based on NAD27 compared to the more accurate NAD83.

    North American Datum of 1983 (NAD83)
    TerraServerUSA data are based on NAD83. Many technological advances in surveying and geodesy since the establishment of NAD27-electronic theodolites, GPS satellites, Very Long Baseline Interferometry, and Doppler systems-revealed weakness in the existing network of control points. Differences became particularly noticeable when linking existing control with newly established surveys. The establishment of a new datum would allow for a single datum to cover North America and surrounding areas, consistently.

    The North American Datum of 1983 is based upon both Earth and satellite observations, using the GRS80 spheroid. The origin for this datum is the Earth's center of mass. This affects the surface location of all latitude-longitude values enough to cause locations of previous control points in North America to shift, sometimes as much as 50 feet. A ten-year multinational effort tied together a network of control points for the United States, Canada, Mexico, Greenland, Central America, and the Caribbean.

*The preceding definitions are adapted from the Georgia State GIS Data Clearinghouse website. To learn more about projections, datums, or coordinate systems refer to:  Peter H. Dana, The Geographer's Craft Project, Department of Geography, The University of Colorado at Boulder.

 

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