Coordinate Systems

The "scene" or "local" coordinate system is the most basic coordinate system. The "scene" coordinate system is essentially an arbitrary, Cartesian coordinate system. By definition, the coordinate system is a "flat earth" system that uses linear X, Y and Z coordinates to locate elements with respect to the coordinate systems origin. This coordinate system is best used for smaller area extents where the curvature of the earth is not a concern (less than 4 km).

Although the coordinate system is itself arbitrary, the "scene" coordinate system still supports geolocation because the origin of this arbitrary coordinate system is tied to a geolocation. When translated to this geolocated "tie point", the scene +Y axis corresponds to North and the scene +X corresponds to East. This makes the scene coordinate system an "East North Up" (ENU) coordinate system.

Because the scene coordinate system assumes a flat earth, it is not a good coordinate system to use over large distances. For example, unless your platform is directly overhead, using the scene coordinate system for a high altitude or space-based platform would not be suggested. However, it is a convenient coordinate system for small areas because it allows for the use of Euclidean geometry.

The "geodetic" coordinate system is known to most as the "Latitude and Longitude" coordinate system. The geodetic grid for the planet is comprised of parallel East/West lines of latitude and North/South lines of longitude that intersect at the poles. Latitude and longitude lines are labeled by the angle they subtend with respect to a reference. For latitude, that 0 reference is the Equator and for longitude that 0 reference is the Prime Meridian.

Since the longitude lines are not parallel, the horizontal distance for a degree of longitude depends on your location. Therefore, the geodetic location does not have intuitive understanding of distance that other coordinate systems have. However, the geodetic coordinate system is globally consistent and therefore is a good coordinate system for positioning high altitude and space-based platforms.

In DIRSIG 4.5.0, the new <location> schema was introduced to describe a location in a DIRSIG input file. The following is an example of a geodetic coordinate:

The Universal Transverse Mercator (UTM) coordinate system is a geographic coordinate system which utilizes a conformal projection that preserves angles locally. This is achieved by breaking the entire globe into a sequence of small conformal projections. These local projections are called "grid zones" which are defined by a longitude "zone" (the earth is divided into 60 zones, each 6 degrees wide and number sequentially) and a latitude "band" (the earth is divided into 20 latitude bands, each 8 degrees wide and labeled with a lettering scheme). The horizontal location offsets within a given grid zone are referred to as the "Easting" and "Northing" and are generally measured in meters.

The key advantage of the UTM coordinate system is that distances and angles can be computed using Euclidean geometry over short distances. This makes is very easy to compute distances and angles. And coordinate units are in meters, making the coordinates intuitive to interpret. However, the UTM projections suffer from ambiguities at the meeting of two grids zones and general distortions for large areas and distances. Although UTM could be used to position a plane or satellite, as it moved from one grid zone to another, the relative position will be prone to errors.

In DIRSIG 4.5.0, the new <location> schema was introduced to describe a location in a DIRSIG input file. The following is an example of a UTM coordinate:

The Earth-Centered, Earth-Fixed (ECEF) coordinate system is also known as the "conventional terrestrial" coordinate system. It is a simple Cartesian coordinate system with the center of the earth at it’s origin. The +X axis passes through the Equator and Prime Meridian intersection. The +Z axis passes through the North Pole. The +Y axis is orthogonal to +X and +Z. As a result, this coordinate system rotates with the earth. The distances used along each axis are meters.

Since the entire ECEF reference frame rotates with the earth, this coordinate system is useful for positioning geo-stationary objects such as satellites. In fact, the Global Positioning System (GPS) uses ECEF as it’s primary coordinate system and derives all other coordinates from it. However, since ECEF has an origin that is very far from most locations on the surface of the earth, it would be awkward for a small scene with a platform located at a small distance away because all the coordinates will be biased by the large offset to the center of the earth.

In DIRSIG 4.5.0, the new <location> schema was introduced to describe a location in a DIRSIG input file. The following is an example of an ECEF coordinate:

The choice of a coordinate system is dependent on a host of variables. However, the choice of which coordinate system to use can be customized based on the element to be positioned. It is important to recognize that the inherently geolocated coordinate systems do not have an advantage over the scene coordinate system. For example, do not feel compelled to use a geolocated coordinate system for positioning scene elements because you fear that it will limit what coordinate system you plan on using for the platform positioning. DIRSIG will automatically establish an internal coordinate system for your simulation and translate all coordinates into and out of it. Therefore, the first rule is to use whatever is most convenient (e.g. "use what you have"). Consider the following general recommendations.