Thursday, March 21, 2013

Object Data

What is Object data?
The drawing object represents the geographical location of the real world feature. Apart from the geographic location, a feature may have aspatial information associated with it. For example, a post box may have data such as post code or mail collection time associated with it. You can store such non-geographic data corresponding to the object in an object data table.

Thursday, March 14, 2013

Coordinate Reference Systems

Coordinate Reference system (CRS)
A CRS defines how an object relates spatially to the locations on the Earth’s surface. Using a coordinate reference system, you can integrate multiple datasets with different CRS into your project. Assigning appropriate coordinate system is the most essential part of the data preparatory work. 

A CRS associates a coordinate system with an object by means of a datum. The definition of a CRS must encompass a definition of a coordinate system and a datum. Everest 1830, NAD 83 and WGS 84 are some of the commonly used datum. 

Geographic data can be represented using the Geographic coordinate system in terms of latitude, longitude, and height. Projected coordinate systems are derived from geographical coordinate systems by means of map projections. Map projections always result in Cartesian coordinate systems.

The Prime Meridian located at the Royal Observatory, Greenwich is used as a reference point for measuring Longitudes or East/West angles. Longitude (ʎ) of a point is defined as the angle from the prime meridian to the meridian plane of a given point while latitude (Ø) is the angle between the equatorial plane and the perpendicular to the ellipsoid through a given point. The figure above shows the latitude and longitude of point P.