Orthophotography Definitions

These definitions were provided by Ron Platt from the City of Tucson with additional editing by Pima County GIS.

Elevation Datum

Elevations are referenced to specific vertical datums, or starting points where an elevation value is zero, such as at sea level. The sea level reference datum used in the United States may be the 1929 datum or the ellipsoid for 1988 datum. Numeric values representing elevations within these two datums are different at the same point on the earth's surface. Elevation datums may also be an assumed elevation at a given point for a specific project, such as calling a finished floor elevation 100. This practiced is not encouraged within the GIS structure. Elevation data regarding a specific point is usually referred to as the "Z value".

Horizontal Datum

Horizontal locations of a point are referenced to either latitude and longitude ("x" and "y value" where x represents the east-west direction and y represents the north-south direction within a coordinate system, such as the state plane coordinate system), or where location is based on a relative coordinate system, such as calling the southwest corner of a project North 10,000 and East 10,000.". Local coordinate systems within the GIS structure are discouraged. In the United States, either the 1927 or the 1983 projection systems are used. The latitude and longitude of a point within these two datums may be identical, but the State Plane coordinate values will be different.

DEM

DEM means "Digital Elevation Model". It is a "grid" of points (usually on an even spacing) that contains both horizontal and vertical data about the earth's surface. DEMs are commonly used for the creation of "ortho" photos. A sketch of a DEM is shown below:

DTM

DTM means "Digital Terrain Model". A DTM is a DEM where "breakline" data has been added (breaklines are lines in the topography where grade changes exist, such as tops and toes of slopes) and points have been removed for polygons or other dense features. A sketch of a DTM is shown below:

LIDAR

Light Detection and Ranging (LIDAR) is a remote sensing system used to collect topographic data. These data are collected with aircraft-mounted lasers capable of recording point elevation measurements at a rate of 2,000 to 5,000 pulses per second. The LIDAR sensor records the time difference between the emission of the laser beam and the return of the reflected laser signal to the aircraft. This time difference in conjunction with aircraft mounted GPS positioning is used to determine the coordinates and elevation of each collected point.

Orthophoto

A photograph that has been enhanced such that features on the map have been registered to either a DEM or a DTM in a way that allows for accurate measurements of features and relationships between features, directly on the photograph. This means the map can be scaled in any direction.

Digital Orthophoto

An orthophoto in digital (electronic) form.

Georeferenced

Referencing to the earth's surface, such as latitudes/longitudes, or state plane coordinates.

Grayscale Photography

Black and white photography. Also called Panchromatic.

Multispectral Imagery

Photography where separate images of the same view are gathered. Each image represents a different light wavelength, such as the red, green, blue, and infrared bands. When multispectral bands are combined in different combinations, color enhancements can be achieved to view physical conditions the human eye cannot see. The 1998 Regional Orthophoto Project multispectral imagery has the red, green, blue and near-infrared bands.

Resolution

The pixel size that makes up an image.


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Geographic Information Systems

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