Data Sharing:-The National Pipeline Mapping System
Map Bases
In some cases the originating survey data was extremely accurate, but the data was
'adjusted' to a map base with variable accuracy. Some of the systems do not retain the
original coordinates, but recalculate the values based on the location relative to the map
base. Thus, this same data imported and referenced to a fully rectified orthophoto base
or different resolutions of quad bases, the relative positional accuracy of the information
is significantly degraded.
A consideration with the NPMS data is also that a significant representation of the
pipeline operators did not have GIT databases at their disposal to assist with submission
of the data. Therefore, it may have been hand drawn on quad maps and digitized in one
of the state or national repositories from those records, or digitized from alignment
drawings and warped or rubber sheeted to fit a quad map base.
As for other public data sets, many government entities use the TIGER maps as their base
map, which is appropriate for their needs. However, a very accurate pipeline data set,
imported and overlaid onto a TIGER base may create some significant errors in the
relative positional display of the linear features.
Conversion from Alignment Drawings
A CAD drawing is a two dimensional plane. Some companies have been known to
'stretch out' the slope of a pipeline so that engineers would have the capability to scale
actual distances along the pipe on the drawings. This is most problematic for a GIS
technician. The length calculation algorithm in most GIS software returns a value for the
horizontal map distance, therefore the slope chain measurement along the top of pipe
must be accounted for in the database somehow. Reconciling the drawings with the
actual pipe lengths can be a challenge to say the least during a conversion effort.
These CAD alignment drawings were the primary source of engineering data as well as
the positional information on the pipeline for the past 50 years or more. The scale and
accuracy varies greatly depending on the organization and the industry. Standards and
specifications evolve and change through the years. This method has served the industry
well. However, the conversion process to an enterprise GIS will result in the
scrutinization of all documents and reconciling to a consistent environment.
GPS - 'X' does not mark the spot
If one is fortunate enough to receive data from an organization that was an early adopter
of high quality GPS survey methods, the survey standards and specifications must be
considered. When performing due diligence investigation on a spatial data set, do your
best to also get a copy of the engineering standards and specifications for the period that
the particular data was assembled as companies will change and evolve their standards
over time.
It should be noted however, that some organizations did allow the gathering of coordinate
information from their field units using the handheld GPS units while selective
availability was still in effect. When selective availability was removed in May, 2000
this became much less of an issue - as long as you are aware that most handheld units
provide an accuracy level of approximately 15 meters. This is certainly acceptable for
certain high level analysis of the data. Data gathered via handheld units prior to May,
2000 may only provide a proximal location within 100 meters or more.
One organization may have defined accuracy tolerances for their survey data that is more
or less stringent than another. This information is not always obvious from the metadata
alone.
There is also a danger that an organization may have defined very rigorous accuracy
standards for the incoming survey data, but placed the data into a mapping system with a
base map that is inappropriate for and not capable of supporting the more accurate
coordinate data. Hence, a very accurate coordinate position is severely degraded in order
to show the correct relative position to ground features on the base map.
Summary
Please don't misunderstand that this is meant as a judgment of such data as bad or
inferior. This is not the case at all. In many cases it is the best available at this time.
Most pipeline companies are making Herculean efforts to convert this data to an
enterprise tool that will facilitate more functions and greater access to this critical data.
However, due to those challenges discussed here, it is an ongoing process. The data that
is available is the best that can be presented at this time with the resources available.
Nearly every organization has recognized the issues and has implemented a process of
continuous improvement.
This information is offered with the utmost love and respect for all the organizations
making the effort to evolve to the world of spatial data. These companies have a vested
interest in the accuracy and completeness of this information and its appropriate use.
There are real issues with the misuse of this data, accidental or otherwise. The
considerations put forth here are offered in the sincere hope that the wealth of
information such as that contained in the National Pipeline Mapping System can be fully
realized. There is genuine substantial value as long as the data mines and the tools are
utilized properly with full awareness of the advantages as well as the limitations of both.
For additional information:
United States Department of Transportation, Office of Pipeline Safety
Research and Special Projects Administration
National Pipeline Mapping System
www.npms.rspa.dot.gov
Federal Government Data Committee Metada Standards
www.fgdc.gov
