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.

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.  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:



World file parameters are always stored in this order:

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


D. Rotation in X direction (assumed = 0)


B. Rotation in Y direction (assumed = 0)


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


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


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


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. 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 a scanned image, see the tutorial for using fGIS and HyperCube.

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.
  • The Wisconsin Department of Natural Resources  uses a statewide Wisconsin Transverse Mercator projection/coordinate system, based on the current high-resolution reference network (the 1991 adjustment to the North American Datum of 1983). A shorthand term for the version of WTM based on the 1991 adjustment to NAD83 is "WTM83/91."  Unlike UTM, WTM does not divide Wisconsin between zones. WTM83/91 parameters are:

Grid Coordinate System Name:

WTM83/91 (Wisconsin Transverse Mercator, referenced to the North American Datum of 1983, 1991 adjustment)

Map Projection Name:

Transverse Mercator

Longitude of Central Meridian:

-90 degrees West

Scale Factor at Central Meridian:


Latitude of Projection Origin:

0 degrees

False Easting:


False Northing:




Horizontal Datum:

NAD83/91, also known as HARN or HPGN (1991 HARN corrections are generally less than a half-meter compared to NAD83 in Wisconsin.)

  • 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)
    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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