Don't guess about your GIS Database

Principal of good data management is to obtain the coordinate system information from the data source providing the data. Do not guess about the coordinate system of data because this will result in an inaccurate GIS database.

The necessary parameters are the following:

• Geographic coordinate system (Datum)


• Unit of measure


• Zone (for UTM)


• Projection


• Projection parameters

Projection parameters may be required, depending on the map projection. For example, the Albers and Lambert projections require the following parameters:

• 1st standard parallel


• 2nd standard parallel


• Central meridian


• Latitude of origin


• False easting


• False northing


• Unit of measure

The geoid is an equipotential, or level, surface of the earth’s gravity field. Imagine the oceans allowed to settle under the influence of gravity only and not subject to tidal or atmospheric forces. Tunnels are also used to connect the oceans so that the water can move freely. The resulting surface is a representation of the geoid. The geoid is approximately equal to mean sea level (MSL) and generally differs from local mean sea level by a meter or so. It is a complex shape. The geoid is influenced by the composition of the earth so it may have discontinuities in its slope. This means that the surface is an analytic surface as opposed to a mathematical surface like an ellipsoid. The geoid generally differs from an earth-centered horizontal geodetic datum by less than 100 meters. For example in the United Kingdom, the geoid and the horizontal datum (OSGB36) differ by less than five meters.

Coordinate systems, also known as map projections, are arbitrary designations for spatial data. Their purpose is to provide a common basis for communication about a particular place or area on the earth's surface. The most critical issue in dealing with coordinate systems is knowing what the projection is and having the correct coordinate system information associated with a dataset.

There are two types of coordinate systems—geographic and projected.

A geographic coordinate system uses a three-dimensional spherical surface to define locations on the earth. It includes an angular unit of measure, a prime meridian, and a datum (based on a
spheroid). In a geographic coordinate system a point is referenced by its longitude and latitude values. Longitude and latitude are angles measured from the earth's center to a point on the earth's surface. The angles often are measured in degrees (or in grads).

A projected coordinate system is defined on a flat, two-dimensional surface. Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. A projected coordinate system is always based on a geographic coordinate system that is based on a sphere or spheroid.
In a projected coordinate system, locations are identified by x,y coordinates on a grid, with the origin at the center of the grid. Each position has two values that reference it to that central location. One specifies its horizontal position and the other its vertical position.