IMAPS Viewer Coordinate System Help

Coordinate Systems:

Coordinate Systems are the system of numbers that represent locations, in this case on the Earth. They can represent 2-dimensional (X/Y) or 3-dimensional (X/Y/Z) space. Two common Coordinate Systems used in GIS are Geographic (Latitude/Longitude) and UTM (Universal Transverse Mercator).

Geographic coordinates are in Degrees/Minutes/Seconds units and can be represented many ways, but commonly as Decimal Degrees (DD, eg. 122.3765) or as full Degrees/Minutes/Seconds (DMS, eg. 122 22 35). Geographic coordinates are spherical coordinates and are usually the only coordinates that are applied to an actual globe. There is no Projection that uses Geographic coordinates.

Coordinate Systems are the system of numbers that represent locations, in this case on the Earth. They can represent 2-dimensional (X/Y) or 3-dimensional (X/Y/Z) space. Two common Coordinate Systems used in GIS are Geographic (Latitude/Longitude) and UTM (Universal Transverse Mercator).

Geographic coordinates are in Degrees/Minutes/Seconds units and can be represented many ways, but commonly as Decimal Degrees (DD, eg. 122.3765) or as full Degrees/Minutes/Seconds (DMS, eg. 122 22 35). Geographic coordinates are spherical coordinates and are usually the only coordinates that are applied to an actual globe. There is no Projection that uses Geographic coordinates.

UTM coordinates are typically in meters, but can also be in feet, and the system is a Projection as well as a coordinate system.

The UTM system divides up the Earth into vertical zones that each have a unique axis center point from which the coordinate system starts. In California, any point west of 120 degrees (the CA/NV border in the north) is in Zone 10, while any point east of 120 degrees is in Zone 11.

Projections:

Projections convert spherical Earth coordinates (latitude/longitude) to planar coordinates that can be represented on a flat map.

All projections create some level of distortion of shape, area and distance, as there is no perfect way to flatten a spherical shape. Projections are chosen based on what is needed for a given application: true shape, true area, or true distance.

Datums:

A Datum is the model that is used by a projection to represent the shape and size of the Earth. Datums have evolved from those describing a spherical Earth to ellipsoidal models derived from years of satellite measurements. Referencing coordinates to the wrong datum can result in position errors of hundreds of meters.

Two common datums for North America are NAD 1927 and NAD 1983.
NAD27 (North American Datum of 1927) is a old system derived from land-based surveys.
NAD83 has gradually replaced NAD27 and is a satellite-based system using the center of the Earth as a reference point using measurements taken in 1980.

The offset between these Datums vary based on location, but can be as much as several hundred meters.

WGS 1984 is a common default datum used in GPS units, and is equivalent to NAD 1983 (slight differences of no significance for practical applications). The WGS 1984 spheroid is almost identical to the GRS 80 spheroid which was used for NAD 1983. Because both are so close, NAD 1983 is compatible with GPS data. The raw GPS data is actually reported in the WGS 1984 coordinate system.


Addition links:

Projections, Coordinate Systems, and Datums Explained: NOAA
Coordinate Systems Overview (detailed): The Geographer's Craft
Map Projections explained (basic): about.com
NAD83 vs WGS84: NOAA paper, and NOAA docket