Coordinate Systems
XMS programs work in a single coordinate system. Using the Edit | Coordinate System menu command selects what system will be used.
Both a horizontal and vertical system are specified. Many numerical models work in a local system, so there may not be a problem if it is not known what coordinate system the data is referenced to. If the desired model requires a global system, or the base data is referenced to more than one system, the modeler must determine the coordinate systems involved.
Since data can be gathered and referenced to various coordinate systems, XMS allows converting data from one coordinate system into another. The Coordinate Conversions page describes how to do this. The available systems and units include:
Horizontal System
The options to the right of the dialog change as the horizontal system is changed. For example, the hemisphere is required for a Geographic system and a UTM zone is required for a UTM system.
Local is the default horizontal and vertical system. Unless desiring to convert data to another system, it is not necessary to change the system from Local.
Horizontal Units
The horizontal units can be specified for all systems except Geographic, which uses decimal degrees. The units set here are the units used by the models such as RMA2.
- U.S. Survey Feet
- International Feet
- Meters
Ellipsoid
The Ellipsoid options are only available for non-NAD/HPGN/CPP systems. The ellipsoid can be selected for a region of the world. Changing the ellipsoid changes the minor and major radii of the earth, measurements used in performing conversions. To set the radii for an ellipsoid not included in XMS, select the User Defined ellipsoid and a button appears titled Define. Clicking on the button brings up a dialog where the radii are defined. The Major and Minor Radii (a and b, respectively) are defined in meters only. The Minor Radius can be input directly or defined by specifying the ellipsoid flattening (1/f) or eccentricity squared (e2) variables, where
Vertical System
The supported vertical options are for North America. If a non-NAD/HPGN horizontal system is selected, this option is disabled.
- Local
- A system defined for a survey that may or may not relate to a global coordinate system. For more information, see Local Coordinate System
- NGVD 29
- NGVD 29 (The National Geodetic Vertical Datum of 1929) is a vertical geodetic datum formerly called "Sea Level Datum of 1929" or "mean sea level". The datum was found by averaging the sea level over a period of many years at 26 tide stations along the coasts of the US and Canada. Because it is an average, it does not represent the local mean sea level at any particular place. Conversion to NAVD 88 is done by interpolating between data points between the two data.
- NAVD 88
- NAVD 88 (The North American Vertical Datum of 1988) is a vertical geodetic datum created in 1991 from measurements in Mexico, the US, and Canada. NAVD 88 was computed due to land distortions over time from the NGVD 29 measurements. Conversion to NGVD 29 is done by interpolating between data points between the two data.
Vertical Units
The vertical unit is only used for conversions. Area and volume calculations performed by XMS use the horizontal units.
- U.S. Survey Feet
- International Feet
- Meters
See also
External Links
- Introduction to Tidal and Geodetic Vertical Datums and Datum Computations – Presentations available at ftp.ngs.noaa.gov
