Watershed Assessment, Tracking & Environmental ResultS

RAD Spatial Data

The RAD is hosted in an Oracle Enterprise database.  Within the database, the spatial component of RAD data is stored using ESRI ArcSDE and Oracle Spatial technologies.  Each spatial dataset is stored twice - once as ArcSDE registered ESRI ST_GEOMETRY spatial type for Oracle and a second time as ArcSDE registered Oracle Spatial SDO_GEOMETRY.  This provides the maximum flexibility and performance considerations for the variety of services and clients consuming the RAD.  This applies to RAD event tables, NHD tables, and associated hosted datasets such as NHDPlus.

The historical convention is to indicate the storage type via a suffix of "_SDE" for ESRI ST_GEOMETRY and "_OS" for Oracle SDO_GEOMETRY. 

For more information on these technologies, please review the vendor documentation:

• ESRI ArcSDE
  
  
• ESRI Geodatabase in Oracle
   
  
• Oracle Spatial
  
• Oracle Spatial Users Guide 

Coordinate Reference Information

All spatial data regardless of storage type is stored in an unprojected NAD83 coordinate space.  For Oracle Spatial SDO_GEOMETRY objects, the SDO_SRID value used in all geometries is 8265 defined by Oracle Corporation as "Longitude / Latitude (NAD 83)".  This is equivalent to SDO_SRID 4269 as defined by the OGP Surveying & Positioning Committee .  For ESRI SDE.ST_GEOMETRY objects, the SRID value used is 4269 defined by ESRI as "GCS_North_American_1983".  Note that the ST_SRID value stored directly within SDE.ST_GEOMETRY objects does not reference any universal standard of coordinate systems but is instead a lookup value specific to the instance of ArcSDE hosting the layer. 

Angular Unit: Degree (0.017453292519943299)
Prime Meridian: Greenwich (0.000000000000000000)
Datum: D_North_American_1983
Spheroid: GRS_1980
Semimajor Axis: 6378137.000000000000000000
Semiminor Axis: 6356752.314140356100000000
Inverse Flattening: 298.257222101000020000

Linear Referencing Systems in RAD and NHD

Point and linear events in the RAD are reach indexed with the use of linear references stored within RAD and NHD geometries.  This system, also known as dynamic segmentation, adds an additional length or measure dimension to all spatial coordinates.  The measurement system can be in any desired units (for the purposes of the NHD it is defined as a percent of the NHD reach).  Using LRS functions native to Oracle Spatial and ESRI applications, users may store, display, query, and analyze event information without segmenting or breaking the linear feature.   Therefore multiple sets of event information can be associated to any part of a feature without modification of the base coordinate data.

ESRI Dynamic Segmentation

Oracle Linear Referencing System 

Measurements in RAD and NHD datasets

All length and area calculations generated and stored in the Reach Address Database follow established USGS standards for the National Hydrography Dataset.  While geometries are stored and manipulated as NAD83 geodetic data, when length and area are calculated the geometries are first projected into an Albers Equal-Area Conic projection having the following parameters:

Projection: Albers
False_Easting: 0.0
False_Northing: 0.0
Central_Meridian: -96.0
Standard_Parallel_1: 29.5
Standard_Parallel_2: 45.5
Latitude_Of_Origin: 23.0
Linear Unit: Meter (1.0)

Geographic Coordinate System: GCS_North_American_1983
Angular Unit: Degree (0.017453292519943299)
Prime Meridian: Greenwich (0.0)
Datum: D_North_American_1983
  Spheroid: GRS_1980
    Semimajor Axis: 6378137.0
    Semiminor Axis: 6356752.3141403561
    Inverse Flattening: 298.25722210100002

This projection is defined in ESRI products as "USA_Contiguous_Albers_Equal_Area_Conic_USGS_version" and is assigned an ESRI SRID value of 102008.  This projection does not exist currently in the default Oracle projections but can added to your server with this sql script.