Height and Transformations Vertical Datums on-line ngs geoid resources

Height and Transformations

Vertical Datums

ON-LINE NGS GEOID RESOURCES

• Official NGS Geoid Page:

• http://www.ngs.noaa.gov/GEOID
• Links to all geoid models
• Links to all geoid papers from NGS
• Lots of useful geoid information
• Slideshows like this one (coming soon)

Datum Transformation and Coordinate Conversion

Cartesian Coordinate System

Datums and Defining Parameters

• To translate one datum to another we must know the relationship between the chosen ellipsoids in terms of position and orientation. The relationship is is defined by 7 constants.
• A. 3 - Distance of the ellipsoid center from the center of the earth (X, Y, Z)
• B. 3 - Rotations around the X, Y, and Z of the Cartesian Coordinate System Axes (, , )
• C. 1 - Scale change (S) of the survey control network
• 2 - The size and shape of the ellipsoid (semi major axis a and flattening f approximately 1/298

Differences Between Horizontal Datums

• The two ellipsoid centers called  X,  Y,  Z

• The rotation about the X,Y, and Z axes in seconds of arc

• The difference in size between the two ellipsoids

• Scale Change of the Survey Control Network S

Coordinate Conversion & Datum Transformation

• References: NIMA TR8350.2, TEC-SR-7, DMA TM 8358.1

• Convert from Grid to Geographic Coordinates

• Convert from Geographic to Cartesian Coordinates

• Apply Datum Transformation 3 Parameter (X, Y, Z, and a & f of Ellipsoid) 7 Parameter (X , Y , Z , S, a & F of Ellipsoid) * For most uses 3 parameter shifts are acceptable

• Compute New Geographic Coordinates

• Compute New UTM, MGRS etc. Coordinates

• Error in transformation propagates to final coordinates

Coordinate Conversion & Datum Transformation

Geographic Translator (GEOTRANS)

• Converts coordinates among a wide variety of coordinate systems, map projections, and datums.

• 11 different coordinate systems and map projections
• Over 200 different datums
• Choose between Ellipsoid and MSL height
• Program can receive coordinates from a text file, convert them, and output results in another file.