Feature: Alliance for New Heights
Professional Surveyor Magazine - July 2012
Two Cochise County staffs—survey and GIS—collaborate in rural Arizona to establish and maintain a mutual vision of success.
By Walter Domann
The alliance of GIS and survey staffs has a long history at Cochise County, Arizona. It started when, as the GIS coordinator, I began discussing the importance of collaboration with Dave Sutherland, the then-County surveyor. Our shared vision was a mutual commitment to integrating and advancing the use of geospatial technologies such as GIS and GNSS to increase efficiencies and effectiveness within county government. Collaboration was how we advanced our vision.
As a result of frequent debate and discussion, interaction, and dedication to implementing certain geospatial technologies and services, the survey and GIS staffs achieved a much better understanding of each others’ day-to-day workflow and responsibilities as well as an awareness of how closely the two staffs are interrelated and interdependent (in fact, we started referring to ourselves as just one staff). By working together toward common goals such as developing a geodetic control network, using a low-distortion map projection (LDP), and integrating land-record research documents into the system, the survey and GIS staffs became more successful in their respective roles and responsibilities.
Collaboration Begins with the Geodetic Control Network
In 2003, Sutherland embarked on an ambitious effort to upgrade the geodetic control network in Cochise County. He enlisted the support and expertise of Dave Minkel, National Geodetic Survey (NGS) geodetic advisor to Arizona, to help implement the plan. Sutherland realized the GIS was the most effective tool to use for the analysis, so he leveraged the experience of the County GIS staff to quickly acquire the appropriate geospatial data and then provide the essential cartographic services to support the decision-making process.
After the analysis was completed for the primary network of 17 existing stations, we developed a plan that included three new control stations that met the criteria for stability, position order, condition, and location. Over 800 existing stations were included in the original analysis, but most were eliminated because of the factors of stability and condition; the remaining stations were chosen for further analysis because of their accessibility and spatial relationship to the other stations.
After the network construction phase, survey and GIS staffs from public and private organizations occupied stations for the multi-day static GPS sessions per NGS Height Modernization guidelines NGS-58 and NGS-59 (available at www.geodesy.noaa.gov/PUBS_LIB/NGS-58.pdf
). Sutherland coordinated these sessions through his professional associations and affiliation with Arizona Professional Land Surveyors.
The first multi-day GNSS static sessions came to be known as Operation ZOMBIE. The project title, Z values Obtained Mainly by Incomparable Employees, was created by Jodi McGrath (GIS analyst) and unanimously adopted by survey and GIS staffs (the final day of the static sessions occurred on Halloween).
Over the next three years, four more phases in the control network densification process took place. Additional stations in the control network were required to minimize RTK rover base lines to adjacent stations for quality control and to maximize reliable radio communications. At each phase in the process, the survey and GIS staff worked together to construct the new control stations. Members of both groups worked together to excavate holes and mix and finish concrete for the new control stations and static GNSS sessions of each new station brought into the network.
For processing of the network, all vector reductions and adjustments were performed by NGS or Geodetic Analysis, LLC using NGS software PAGES and ADJUST in accordance with NGS “bluebooking” procedures. The resulting control coordinates (latitude, longitude, ellipsoid heights, and GNSS-derived orthometric heights) were published in the NGS Integrated Data Base and are available to the public through the NGS website (www.geodesy.noaa.gov
The procedures and methods that comprised the development and advancement of the geodetic control network are the foundation for the collaboration between the survey and GIS staffs that exists today. The existing network of one active control station (CORS) and 126 passive control stations (published) is still vital to the services, including photogrammetric control, of the survey and GIS staffs and private-sector surveyors. Given the size of the county (6220 sq. miles), areas of limited cell phone coverage, and steep terrain that obstructs radio communications, the network of passive control stations will continue to be important for controlling the collection of local survey and GIS data.
Solving the Grid-to-Ground Problem
In 2006, Sutherland realized that the standard GIS State Plane Coordinate System was not going to perform well enough for survey jobs. The grid-to-ground transformation to remove the distortion for survey jobs was too cumbersome for each job, and integrating GNSS data collected directly into the GIS was a challenge.
In order to address this challenge, he enlisted the support and expertise of Michael Dennis, of Geodetic Analysis, LLC, and current NGS geodesist, to develop a Low Distortion Projection (LDP) for Cochise County. The purpose of the LDP was to minimize map projection linear distortion—that is, to create a system where “grid” distances are essentially equal to “ground” distances.
According to Dennis, “The linear distortion issue faced by Cochise County is well known to surveyors as the ‘grid-to-ground’ problem. State Plane grid distances are usually shorter than the true horizontal ground distances. Although the amount of distortion varies with location and,
especially, elevation, in the non-mountainous areas of Cochise County the State Plane grid distances are typically too short by one to two feet per mile. The county wanted a coordinate system that yielded projected (grid) coordinates with low distortion, and they wanted that system compatible with GIS. An LDP satisfies both of these requirements because it is a rigorously designed coordinate system that minimizes linear distortion and is compatible with most GIS, surveying, and engineering software (including field equipment software).
“The Cochise County LDP is a Transverse Mercator projection, which is probably the most widely supported conformal map projection,” said Dennis. “A conformal projection is one where the scale error is the same in all directions, which is an essential characteristic for minimizing linear distortion. The Cochise County LDP has a central meridian scale factor that essentially puts the projection surface ‘at ground’ (which minimizes linear distortion), and the central meridian is located near the midpoint of the county (which minimizes angular distortion).
“In addition, the projected coordinate values were chosen such that they do not equal Arizona State Plane East Zone values anywhere within the county (to avoid accidentally confusing them with State Plane values),” Dennis continued. “The net effect is that the LDP allows determination of low-distortion coordinates directly from surveying observations without having to apply any rotation, translation, or scaling (i.e., without needing a so-called horizontal ‘localization’ or ’calibration’). Because the LDP is well defined, the survey results can be directly imported to (and exported from) a GIS without any transformation. In essence it is like a mini State Plane zone, and like State Plane it is referenced to the National Spatial Reference System, in this case the North American Datum of 1983 (NAD 83).”
The Cochise County LDP complies with the National Spatial Reference System and is used to generate horizontal grid coordinates that reduce the grid-to-ground distortion for the majority of the county to +/-0.2 ft per mile (±40 parts per million). The County GIS quickly adopted the new LDP, and it became the standard for all GNSS data collection and geospatial data reference. The survey and GIS staffs use this coordinate system for all GNSS data collection, which is then integrated into the GIS without any additional scaling, rotation, or translation. They do so by entering the specific LDP parameters into the GNSS data controllers. LDP grid values are also assigned to geodetic control network stations to ensure consistency in GNSS field collection data. The LDP allows for seamless GNSS data collection and integration into the GIS where observed ground distances are consistently within tolerances for survey jobs.
Although ellipsoid heights are not part of the LDP definition, they affect linear distortion because distortion is in part a function of height above the reference ellipsoid. Because of this, it is important that the LDP be referenced to the NAD 83 datum in order to get NAD 83 ellipsoid heights. Likewise, orthometric heights (i.e., “elevations”) are also not part of the LDP definition. But it is beneficial to use orthometric heights referenced to the North American Vertical Datum of 1988 (NAVD 88) because NAVD 88 is related to NAD 83 ellipsoid heights by NGS hybrid geoid models (such as GEOID09). These relationships between the geodetic and vertical datums facilitate combination of the two-dimensional LDP with one-dimensional heights to create fully integrated three-dimensional spatial datasets.
Partnership for Documentation
Research and recovery of PLSS, mineral survey, and property monumentation is a time-consuming but critical process for survey and GIS staffs. To address that challenge, the two staffs worked together to develop a list of important documents that are most frequently referenced during the research process. As a result of that partnership, we have integrated over 28,000 scanned documents into the GIS.
The list of document types that can be accessed through the GIS includes recorded subdivision plats, records of survey, GLO notes and plats, BLM surveys and master title plats, railroad ROW and track maps, mineral surveys, and homestead entry surveys. You can access the documents by clicking on the appropriate map feature, usually the PLSS section or township, and then pick the desired document from the list of documents for that location. This makes the research process and subsequent recovery effort more efficient. Because the County survey crew is an integral part of the land-record maintenance, this efficiency is crucial to maximizing the amount of GNSS field-work data integrated in the GIS.
The data was compiled and the system developed entirely by GIS staff. By request of the Cochise County recorder, all records in the system are available to the public through a Google Earth kmz file that’s maintained by GIS staff.
The Cochise County survey and GIS staffs will continue to work closely together. This crucial collaboration will continue to support the advancement and integration of certain geospatial technologies and the distribution and use of decision-quality geospatial data across departmental and jurisdictional boundaries. The goal is enhancing workflow and providing a more efficient and effective county government for the benefit of the community and the respective professions.
The operational priority will be a continued focus on the fundamentals of using good survey control and the appropriate mapping grid. We will be enhancing the research capability of the GIS with a strong emphasis on sustaining the well-established momentum that has resulted from working cooperatively toward mutual goals for nearly ten years.
Walter Domann is the GIS coordinator for Cochise County in Arizona. He has worked for Cochise County for 17 years. In April 2012 the Arizona Professional Land Surveyors presented Domann with the prestigious APLS Geospatial Professional Award.
» Back to our July 2012 Issue