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4. Spatial Reference



Spatial Reference provides the description of the reference frame for, and the means to encode coordinates in a data set. This section contains definitions and examples for the following metadata elements.

Horizontal Coordinate System Definition
Vertical Coordinate System Definition


Horizontal Coordinate System Definition


Horizontal Coordinate System Definition describes the reference frame or system from which linear or angular quantities are measured and assigned to the position that a point occupies. This metadata element encompasses the following sub-elements.


Geographic

The quantities of latitude and longitude which define the position of a point on the Earth's surface with respect to a reference spheroid.

Latitude Resolution

Definition: The minimum difference between two adjacent latitude values expressed in Geographic Coordinate Units of measure.

Format: Latitude Resolution > 0.0


Longitude Resolution

Definition: The minimum difference between two adjacent latitude values expressed in Geographic Coordinate Units of measure.

Format: Longitude Resolution > 0.0

Geographic Coordinate Units

Definition: Units of measure used for the latitude and longitude values.

Format: Select from the following list.
Geographic Coordinate Units:
Decimal degrees
Decimal minutes
Decimal seconds
Degrees and decimal minutes
Degrees, minutes, and decimal seconds
Radians
Grads


Planar

the quantities of distances, or distances and angles, which define the position of a point on a reference plane to which the surface of the Earth has been projected.
Map Projection

The systematic representation of all or part of the surface of the Earth on a plane or developable surface.
Map Projection Name

Definition: The name of the map projection.

Format: Select from the following list.
Map Projections:
Albers Conical Equal Area
Azimuthal Equidistant
Equidistant Conic
Equirectangular
General Vertical Near-sided Perspective
Gnomonic
Lambert Azimuthal Equal Area
Lambert Conformal Conic
Mercator
Modified Stereographic for Alaska
Miller Cylindrical
Oblique Mercator
Orthographic
Polar Stereographic
Polyconic
Robinson
Sinusoidal
Space Oblique Mercator
Stereographic
Transverse Mercator
van der Grinten
other projection

Example:
Map Projection Name: Lambert Conformal Conic

Map Projection Parameters

The parameters for a specific map projection, each having a unique mathematical relationship between the Earth and the plane or developable surface.

Each map projection has a specific set of defining parameters. The list below provides a reference for identifying the required parameters and syntax for each map projection.
1 Definitions and instructions for completing the metadata for the projection parameters follows this list.

Albers Conical Equal Area (one or two occurrences of) Standard Parallel:
Longitude of Central Meridian:
Latitude of Projection Origin:
False Easting:
False Northing:

Azimuthal Equidistant Longitude of Central Meridian:
Latitude of Projection Origin:
False Easting:
False Northing:

Equidistant Conic (one or two occurrences of) Standard Parallel:
Longitude of Central Meridian:
Latitude of Projection Origin:
False Easting:
False Northing:

Equirectangular Standard Parallel:
Longitude of Central Meridian:
False Easting:
False Northing:

General Vertical Near-sided Perspective: Height of Perspective Point Above Surface:
Longitude of Projection Center:
Latitude of Projection Center:
False Easting:
False Northing:

Gnomonic Longitude of Projection Center:
Latitude of Projection Center:
False Easting:
False Northing:

Lambert Azimuthal Equal Area Longitude of Projection Center:
Latitude of Projection Center:
False Easting:
False Northing:

Lambert Conformal Conic (one or two occurrences of) Standard Parallel:
Longitude of Central Meridian:
Latitude of Projection Origin:
False Easting:
False Northing:

Mercator Standard Parallel: or Scale Factor at Equator:
Longitude of Central Meridian:
False Easting:
False Northing:

Modified Stereographic for Alaska False Easting:
False Northing:

Miller Cylindrical Longitude of Central Meridian:
False Easting:
False Northing:

Oblique Mercator Scale Factor at Center Line:

Oblique Line Azimuth Azimuthal Angle:
Azimuth Measure Point Longitude:

or
Oblique Line Point (two occurrences of both)
Oblique Line Latitude:
Oblique Line Longitude:


Latitude of Projection Origin:
False Easting:
False Northing:

Orthographic Longitude of Projection Center:
Latitude of Projection Center:
False Easting:
False Northing:

Polar Stereographic Straight-Vertical Longitude from Pole:

Standard Parallel:
or
Scale Factor at Projection Origin:

False Easting:
False Northing:

Polyconic Longitude of Central Meridian:
Latitude of Projection Origin:
False Easting:
False Northing:

Robinson Longitude of Projection Center:
False Easting:
False Northing:

Sinusoidal Longitude of Central Meridian:
False Easting:
False Northing:

Space Oblique Mercator (Landsat) Landsat Number:
Path Number:
False Easting:
False Northing:

Stereographic Longitude of Projection Center:
Latitude of Projection Center:
False Easting:
False Northing:

Transverse Mercator Scale Factor at Central Meridian
Longitude of Central Meridian:
Latitude of Projection Origin:
False Easting:
False Northing:

van der Grinten Longitude of Central Meridian:
False Easting:
False Northing:

Standard Parallel

Definition: The line of constant latitude at which the surface of the Earth and the plane or developable surface intersect.

Format: -90.0 <= Standard Parallel <= 90.0

Longitude of Central Meridian

Definition: The line of longitude at the center of a map projection generally used as the basis for constructing the projection.

Format: -180.0 <= Longitude of Central Meridian <= 180.0

Latitude of Projection Origin

Definition: The latitude chosen as the origin of rectangular coordinate for a map projection.

Format: -90.0 <= Latitude of Projection Origin

False Easting

Definition: The value added to all "x" values in the rectangular coordinate for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of measure identified in Planar Coordinate Units.

Format: Free text.


False Northing

Definition: The value added to all "y" values in the rectangular coordinates for a map projection. This value frequently is assigned to eliminate negative numbers. Expressed in the unit of measure identified in Planar Coordinate Units.

Format: Free text.

Scale Factor at Equator

Definition: A multiplier for reducing a distance obtained from a map by computation or scaling to the actual distance along the equator.

Format: Scale Factor at Equator > 0.0

Height of Perspective Point Above Surface

Definition: The height of viewpoint above the Earth, expressed in meters.

Format: Height of Perspective Point Above Surface > 0.0

Longitude of Projection Center

Definition: The longitude of the point of projection for azimuthal projections.

Format: -180.0 <= Longitude of Projection Center < 180.0

Latitude of Projection Center

Definition: The latitude of the point of projection for azimuthal projections.

Format: -90.0 <= Latitude of Projection Center <= 90.0

Scale Factor at Center Line

Definition: A multiplier for reducing a distance obtained from a map computation or scaling to the actual distance along the center line.

Format: Scale Factor at Center Line > 0.0

Oblique Line Azimuth

The method used to describe the line along which an oblique mercator map projection is centered using the map projection origin and an azimuth.
Azimuthal Angle

Definition: An angle measured clockwise from north, and expressed in degrees.

Format: 0.0 <= Azimuthal Angle < 360.0

Azimuth Measure Point Longitude

Definition: The longitude of the map projection origin.

Format: -180.0 <= Azimuth Measure Point Longitude < 180.0

Oblique Line Point

The method used to describe the line along which an oblique mercator map projection is centered using two points near the limits of the mapped region that define the center line.

Oblique Line Latitude

Definition: The latitude of a point defining the oblique line.

Format: -90.0 <= Oblique Line Latitude <= 90.0

Oblique Line Longitude

Definition: The longitude of a point defining the oblique line.

Format: -180.0 <= Oblique Line Longitude < 180.0


Straight Vertical Longitude from Pole

Definition: The longitude to be oriented straight up from the North or South Pole.

Format: -180.0 <= Straight Vertical Longitude from Pole < 180.0

Scale Factor at Projection Origin

Definition: a multiplier for reducing a distance obtained from a map by computation or scaling to the actual distance at the projection origin.

Format: Scale Factor at Projection Origin > 0.0

Landsat Number

Definition: The number of the Landsat satellite.
(Note: This data element exists solely to provide a parameter needed to define the space oblique mercator projection. It is not used to identify data originating from a remote sensing vehicle.
2)

Format: 0 < Landsat Number < 5

Path Number

Definition: The number of the orbit of the Landsat satellite.
(Note: This data element exists solely to provide a parameter needed to define the space oblique mercator projection. It is not used to identify data originating from a remote sensing vehicle.
2)

Format: 0 < Path Number < 251 for Landsats 1, 2, or 3
0 < Path Number < 233 for Landsats 4 or 5

Scale Factor at Central Meridian

Definition: A multiplier for reducing a distance obtained from a map by computation or scaling to the actual distance along the central meridian.

Format: Scale Factor at Central Meridian > 0.0

Other Projection

Definition: A complete description of a projection, not described elsewhere in [these guidelines], that was used for the data set. The information provided shall include the name of the projection, the names of the parameters and values used for the data set, and the citation of the specification for the algorithms that describe the mathematical relationship between the Earth and the plane or developable surface for the projection.

Format: Free text.


Grid Coordinate System

A plane-rectangular coordinate system usually based on, and mathematically adjusted to, a map projection so that geographic positions can be readily transformed to and from plane coordinates.
Grid Coordinate System Name

Definition: The name of the grid coordinate system.

Format: Select from the following list.
Grid Coord. System: Universal Transverse Mercator
Universal Polar Stereographic
State Plane Coordinate System 1927
State Plane Coordinate System 1983
ARC Coordinate System
other grid system


Grid Coordinate System Parameters

Assemble the parameters for the grid system according to the following list. Definitions and instructions for completing the metadata for the Grid Coordinate System follows this list.
Universal Transverse Mercator UTM Zone Number:
Transverse Mercator:
(use parameters from Transverse Mercator)
Universal Polar Stereographic UPS Zone Identifier:
Polar Stereographic:
(use parameters from Polar Stereographic)
ARC Coordinate System ARC System Zone Identifier:
Equirectangular
or
Azimuthal Equidistant
(use parameters from the appropriate projection)
State Plane Coordinate System SPCS Zone Identifier
Lambert Conformal Conic
or
Transverse Mercator
or
Oblique Mercator
or
Polyconic
(use parameters from the appropriate projection)


Universal Transverse Mercator (UTM)

A grid system based on the transverse mercator projection, applied between latitudes 84 degrees north and 80 degrees south on the Earth's surface.
UTM Zone Number

Definition: The identifier for the UTM zone.

Format: Integer; 1 <= UTM Zone Number <= 60 for the northern hemisphere; -60 <= UTM Zone Number <= -1 for the southern hemisphere.

Universal Polar Stereographic (UPS)

A grid system based on the polar stereographic projection, applied to the Earth's polar regions north of 84 degrees north and south of 80 degrees south.
UPS Zone Identifier

Definition: The identifier for the UPS zone.

Format: A, B, Y, Z

State Plane Coordinate System (SPCS)

A plane-rectangular coordinate system established for each state in the United States by the National Geodetic Survey.
SPCS Zone Identifier

Definition: The identifier for the SPCS zone.

Format: Four digit numeric codes for the State Plane Coordinate Systems based on the North American Datum of 1927 are found in:
U.S. Department of Commerce. 1986. Representation of Geographic Point Locations for Information Interchange. (Federal Information Processing Standard 70-1). Washington, D.C.: U. S. Department of Commerce, National Institute of Standards and Technology.

Codes for the State Plane Coordinate Systems based on the North American Datum of 1983 are found in: U.S. Department of Commerce. 1989 (January). State Plane Coordinate System of 1983. (National Oceanic and Atmospheric Administration Manual NOS NGS 5). Silver Spring, Maryland: National Oceanic and Atmospheric Administration, National Ocean Service, Coast and Geodetic Survey.

ARC Coordinate System

The Equal ARC-second Coordinate System, a plane-rectangular coordinate system established in: U.S. Department of Defense. 1990. Military Specification ARC Digitized Raster Graphics (ADRG). (MIL-A-89007). Philadelphia: U. S. Department of Defense, Defense Printing Service Detachment Office.
ARC System Zone Identifier

Definition: The identifier for the ARC Coordinate System zone.

Format: Integer: 1 <= ARC System Zone Identifier <= 18

Other Grid System

Definition: A complete description of a grid system, not defined elsewhere [in these guidelines], that was used for the data set. The information provided shall include the name of the grid system, the names of the parameters and values used for the data set, and the citation of the specification for the algorithms that describe the mathematical relationship between the Earth and the coordinates of the grid system.

Format: Free text.

Local Planar

Any right-handed planar coordinate system of which the z-axis coincides with a plumb line through the origin that locally is aligned with the surface of the Earth.

"A local planar coordinate system is any planar coordinate system for which the relationship between the planar coordinates and geographic (latitude and longitude) coordinates is not known. In these cases, the standards ask the producer to describe the coordinate system (the orientation of the axis, etc.) and any means that can be used to link the local system to geographic coordinates."
2
Local Planar Description

Definition: A description of the local planar system.

Format: Free text.

Local Planar Georeference Information

Definition: a description of the information provided to register the local planar system to the Earth (e.g. control points, satellite ephemeral data, inertial navigation data).

Format: Free text.

Planar Coordinate Information

Information about the coordinate system developed on the planar surface.
Planar Coordinate Encoding Method

Definition: The means used to represent horizontal positions.

Format: Select from list: coordinate pair, distance and bearing, row and column.


Coordinate Representation

The method of encoding the position of a point by measuring its distance from perpendicular reference axes (the "coordinate pair" and "row and column" methods).
Abscissa Resolution

Definition: The (nominal) minimum distance between the "x" or column values of two adjacent points, expressed in Planar Distance Units of measure.

Format: Abscissa Resolution > 0.0

Ordinate Resolution

Definition: The (nominal) minimum distance between the "y" or row values of two adjacent points, expressed in Planar Distance Units of measure.

Format: Ordinate Resolution > 0.0

Distance and Bearing Representation

A method of encoding the position of a point by measuring its distance and direction (azimuthal angle) from another point.
Distance Resolution

Definition: The minimum distance measurable between two points, expressed in Planar Distance Units of measure.

Format: Distance Resolution > 0.0

Bearing Resolution

Definition: The minimum angle measurable between two points, expressed in Bearing Units of measure.

Format: Bearing Resolution > 0.0

Bearing Units

Definition: The units of measure used for angles.

Format: Select from the following list.
Bearing Units: decimal degrees
decimal minutes
decimal seconds
degrees and decimal minutes
degrees, minutes, and decimal seconds
radians
grads


Example:
Bearing Units: degrees and decimal minutes

Bearing Reference Direction

Definition: The direction from which the bearing is measured.

Format: Select from list: North, South

Example:
Bearing Reference Direction: South

Bearing Reference Meridian

Definition: The axis from which the bearing is measured.

Format: Select from list: assumed, grid, magnetic, astronomic, geodetic

Example:
Bearing Reference Meridian: geodetic

Planar Distance Units

Definition: The units of measure used for distances.

Format: Select from list: meters, international feet, survey feet; or free text.

Local Coordinate System

A description of any coordinate system that is not aligned with the surface of the Earth.

"A local coordinate system is any non-planar, non-geographic coordinate system. Examples include oblique photography and unrectified satellite images. In these cases, the standards ask the producer to describe the coordinate system and any means that can be used to link the local system to geographic coordinates."
2
Local Description

Definition: A description of the coordinate system and its orientation to the surface of the Earth.

Format: Free text.

Local Georeference Information

Definition: A description of the information provided to register the local system to the Earth (e.g. control points, satellite ephemeral data, inertial navigation data).

Format: Free text.

Geodetic Model

Parameters for the shape of the Earth.
Horizontal Datum Name

Definition: The identification given to the reference system used for defining the coordinates of points.

Format: Select: North American Datum of 1927, North American Datum of 1983; or free text.


Ellipsoid Name

Definition: The identification given to established representations of the Earth's shape.

Format: Select: Clark 1866, Geodetic Reference System 80; or free text.

Semi-major Axis

Definition: The radius of the equatorial axis of the ellipsoid.

Format: Semi-major Axis > 0.0

Denominator of Flattening Ratio

Definition: The denominator of the ratio of the difference between the equatorial and polar radii of the ellipsoid when the numerator is set to 1.

Format: Denominator > 0.0

Vertical Coordinate System Definition

The reference frame or system from which vertical distances (altitudes or depths) are measured. This metadata element encompasses the following sub-elements.

Altitude System Definition

The reference frame or system from which altitudes (elevations) are measured.
Altitude Datum Name

Definition: The identification given to the level surface taken as the surface of reference from which altitudes are measured.

Format: Select: National Geodetic Vertical Datum of 1929, North American Vertical Datum of 1988; or free text.

Altitude Resolution

Definition: the minimum distance possible between two adjacent altitude values, expressed in Altitude Distance Units of measure.

Format: Altitude Resolution > 0.0

Altitude Distance Units

Definition: The units in which altitudes are recorded.

Format: Select: meters, feet; or free text.

Altitude Encoding Method

Definition: The means used to encode the altitudes.

Format: Select: Explicit elevation coordinate included with horizontal
coordinates, Implicit coordinate, Attribute values.

Depth System Definition

The reference frame or system from which depths are measured.
Depth Datum Name

Definition: The identification given to surface of reference from which depths are measured.

Format: Select from the following list; or free text.
Depth Datum: Local surface
Chart datum; datum for sounding reduction
Lowest astronomical tide
Highest astronomical tide
Mean low water
Mean high water
Mean sea level
Land survey datum
Mean low water springs
Mean high water springs
Mean low water neap
Mean high water neap
Mean lower low water
Mean lower low water springs
Mean higher high water
Mean higher low water
Mean lower high water
Spring tide
Tropic lower low water
Neap tide
High water
Higher high water
low water
Low-water datum
Lowest low water
Lower low water
Lowest normal low water
Mean tide level
Indian spring low water
High-water full and charge
Low-water full and charge
Columbia River datum
Gulf Coast low water datum
Equatorial springs low water
Approximate lowest astronomical tide
No correction.

Depth Resolution

Definition: The minimum distance possible between two adjacent depth values, expressed in Depth Distance Units of measure.

Format: Depth Resolution > 0.0

Depth Encoding Method

Definition: the means used to encode depths.

Format: Select: Explicit elevation coordinate included with horizontal
coordinates, Implicit coordinate, Attribute values.


References



1
For reference information on these map projections and their parameters see: Snyder, John. 1987. Map Projections: A Working Manual. U.S. Geological Survey Professional Paper 1395. Washington, D.C: U.S. Government Printing Office.


2
Federal Geographic Data Committee (FGDC). 1995. Content Standards for Digital Geospatial Metadata. Workbook (March 24). Washington, D.C.: FGDC. pp.4-1 - 4-13.



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