Coordinate Systems & Datums

What is NAD83?

North American Datum of 1983 - official horizontal datum of the United States. Based on GRS80 ellipsoid, geocentric.

What is WGS84?

World Geodetic System 1984 - global reference frame used by GPS. Practically identical to NAD83 at the meter level.

What is NAVD88?

North American Vertical Datum of 1988 - official vertical datum of the United States based on 1988 leveling adjustment.

What is the difference between NAD83 and WGS84?

Differences are typically < 2 meters. NAD83 is fixed to North American plate; WGS84 is truly global and updated regularly.

What is ITRF?

International Terrestrial Reference Frame - most accurate global geocentric reference frame maintained by IERS.

What is a reference epoch?

Specific time to which coordinates are referenced, accounting for tectonic plate motion. NAD83 uses epoch 2010.0 in recent realizations.

What are ellipsoid heights?

Height above the reference ellipsoid measured along the normal. Obtained directly from GPS. Not related to water flow.

What are orthometric heights?

Height above the geoid (mean sea level) measured along the plumb line. Used for engineering and water flow calculations.

What is geoid undulation?

Separation between geoid and ellipsoid at a point. Used to convert: Orthometric height = Ellipsoid height - Geoid undulation

What is GEOID18?

Current NGS geoid model for United States defining relationship between GPS ellipsoid heights and leveling orthometric heights.

What is the State Plane Coordinate System?

System of projected coordinates for the United States with multiple zones designed to limit distortion to 1:10,000.

What projections are used for State Plane?

Lambert Conformal Conic: for states wider E-W Transverse Mercator: for states longer N-S

What is California Coordinate System (CCS83)?

California's State Plane system using 6 Lambert Conformal Conic zones. Official coordinate system for California.

What is scale factor?

Ratio of distance on map projection to corresponding distance on ellipsoid. State Plane designed for SF between 0.9999 and 1.0001.

What is combined factor?

Product of grid scale factor and elevation factor. Used for ground-to-grid coordinate conversions.

What is convergence?

Angle between grid north and true north at a point. Varies with longitude within a zone.

What are ECEF coordinates?

Earth-Centered Earth-Fixed (X, Y, Z) Cartesian coordinates with origin at Earth's center. GPS calculates in ECEF before conversion.

What is HTDP?

Horizontal Time-Dependent Positioning - NGS tool for transforming coordinates between epochs and reference frames.

What is deflection of the vertical?

Angular difference between plumb line and ellipsoid normal. Affects astronomic observations and precise leveling.

What is NADCON?

North American Datum Conversion - NGS tool for transforming between NAD27 and NAD83.

What is VERTCON?

NGS tool for transforming elevations between NGVD29 and NAVD88.

What is UTM?

Universal Transverse Mercator - worldwide coordinate system using 60 zones, each 6° wide. Central meridian scale factor 0.9996.

Why use State Plane over UTM?

State Plane has lower distortion (1:10,000 vs 1:2,500) for surveying and is legally specified for property descriptions.

What is the geoid?

Equipotential surface of Earth's gravity field corresponding to mean sea level. Used as vertical datum reference.

How much does the geoid vary from ellipsoid in US?

GEOID18 shows undulations ranging from about -30m to +5m across continental US.