Choice of Spatial Reference for LRS
Configuration of Linear Referencing System
The choice of spatial reference and units of measurement is a business decision and guided by geographical region, operational practices, integrity management and compliance reporting functions. The GIS leads must decide and specify the Spatial Reference and X, Y, Z Tolerance and Resolution to be used for the core information model objects within ArcGIS Pipeline Referencing (APR). The APR toolbox provides various tools to create and modify an LRS that factors in spatial refence. Specifically, the Create LRS tool offers option to set X, Y, Z Tolerance and Resolution. The tool will default to the Tolerance and Resolution settings and units of the Spatial Reference. However, the parameters can be user defined and changed per requirements. It is recommended to use the suggested 1/10th ratio for tolerance/resolution defaults suggested by ArcGIS.
Following example shows the default settings using USA Contiguous Albers Equal Area Conic WKID 102003. Note that using Geographic Coordinate System WGS 1984 WKID 4326 will default the XY Tolerance and Resolution to Decimal Degrees.
USA Albers GCS WGS 1984
Resulting Spatial Reference settings at the LRS dataset including Projected Coordinate System (PCS), Geographic Coordinate System (GCS) and Vertical Coordinate System (VCS):
Resulting Tolerance and Resolution settings (in Meters per the parameters) at the LRS dataset:
Note that the units can be set to supported units including Kilometers, Meters, Centimeters, Miles, Yards, Feet or Inches in the tool parameters for the same spatial reference; the appropriate X, Y, Z Tolerance and Resolution assigned, will be different from the defaults.
Configuration of LRS Network
Once the LRS is created, the Create LRS Network tool offers option to set the Units of Measure along the Network. The Units can be user defined and changed per requirements (supported units include Kilometers, Meters, Centimeters, Miles, Yards, Feet or Inches). Following example shows the default units of Meters using USA Contiguous Albers Equal Area Conic WKID 102003.
Following example shows creating a second network and defining the units to Feet using the LRS that is using USA Contiguous Albers Equal Area Conic Meters.
APR calculates the required M tolerance and resolution for the network feature class that will align the XY tolerance with that of the M tolerance in the desired unit of measure. The XY and Z tolerance and resolution for the network will come from the centerline of the LRS.
As in the example above, the XY tolerance is 0.001 meters and the M tolerance is set as 0.00328083333333333 for the LRS Network measured in Feet. The XY resolution is 0.0001 meters and the M resolution is set as 0.000328083333333333 for the LRS Network measured in Feet. ArcGIS stores the measure values on each vertex of the polyline M-enabled feature class. This is the geometry model that ArcGIS and APR is based on.
0.001 meters ≈ 0.003280833 feet
0.0001 meters ≈ 0.0003280833 feet
The APR Create LRS Event tool takes the X, Y, Z, M tolerances and resolutions of the LRS network and propagates the settings to all events that are registered to that particular Network. Note that APR supports multiple networks as illustrated by additional examples below.
Following instance shows creating a third network and defining the units to Miles using the LRS that is using USA Contiguous Albers Equal Area Conic Meters.
Following example shows creating a fourth network and defining the units to Centimeters using the LRS that is using USA Contiguous Albers Equal Area Conic Meters.
Pre-defined Geodatabase Schema
Organizations may have a pre-determined data model like Esri’s UPDM or PODS. For users with a pre-defined schema, the appropriate Spatial Reference along with X, Y, Z, M Tolerance and Resolution (per conversion calculations) must be set at the time of creation of the empty geodatabase schema. Specifically, the APR information model objects, the LRS Network(s) and related LRS Events need to have compatible X, Y, Z, M Tolerance and Resolution. The configuration options in the APR toolbox along with understanding of Tolerance and resolution settings for the LRS will allow the user to set the desired combination of Spatial Reference and linear units of measurement.
The following table provides the correct tolerance, and resolution settings for key spatial references using Feet or Meters as units of measurement in your LRS.
Spatial Ref: | GCS WGS84 / NAD83 / NAD27 US FEET | GCS WGS84 / NAD83 / NAD27 METERS | PCS NAD83 UTM Zone 14N US FEET | PCS NAD83 UTM Zone 14N Miles |
XY Resolution | 1e-09 Degree | 1e-09 Degree
| 0.0001 Meter
| 0.0001 Meter
|
XY Tolerance | 8.98315284119521e-09 Degree | 8.98315284119521e-09 Degree
| 0.001 Meter
| 0.001 Meter
|
M Resolution | 0.00036481184328240743 | 0.00011119487222222222
| 0.0003280833333333333
| 6.213699494949494e-08 |
M Tolerance | 0.0032771605464840194 | 0.0009988805323293938
| 0.003280833333333333
| 6.213699494949494e-07 |
Z Resolution | 0.0001 | 0.0001 | 0.0001 | 0.0001 |
Z Tolerance | 0.001 | 0.001 | 0.001 | 0.001 |
If using other spatial reference with different units, ArcGIS Solutions provides tools to calculate tolerances, resolutions and change the geodatabase spatial reference.
Any data translation or re-projection of native Spatial Reference and linear units of measurement to different settings will require appropriate transformations. The units of an established linear referencing system should match the proposed system or a unit conversion factored in. Depending on the process and tools used, users may observe spatial shift and/or length mismatches. However, using appropriate transformations are known to minimize such data discrepancies.