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The importance of saving passive survey marks

October 4, 2023  - By

Back in the late 1980s, as project manager of the new adjustment of the North American Vertical Datum of 1988 (NAVD88), I worked with federal and state agencies to perform geodetic leveling and replace lost benchmarks. One of the reasons for the NAVD 88 project was to address the issue that thousands of benchmarks placed in previous decades had been subsequently destroyed and many others had been affected by crustal motion, postglacial rebound, and subsidence due to the withdrawal of underground fluids. NGS along with its partners performed thousands of kilometers of leveling to replace lost benchmarks. That said, the loss of control marks, denoted by some as “passive marks,” still seems to be a problem today.

California surveying agencies played a part in replacing and updating lost marks for the NAVD 88 project and it seems that they are doing it again. On Sept. 21, the importance of saving passive marks was discussed at the 2023 CLSA Geomatics Conference at Cal Poly Pomona/College of Engineering.

Defining passive marks

Several of my previous columns have highlighted the new, modernized NGS National Spatial Reference System (NSRS), and how active and passive control will be part of the new system.

Active and passive control. (Image: NGS)

Active and passive control. (Image: NGS)

For all practical purposes, passive marks are marks that are not continuously operating reference stations (CORS).

On June 22, NGS held a webinar on the benefits and challenges of transitioning to the modernized NSRS at which the presenters were not NGS employees. Users can download the presentation here.

NGS webinar on June 22. (Image: NGS)

NGS webinar on June 22. (Image: NGS)

I want to highlight a few statements made by Brian Fisher, director of the American Association for Geodetic Surveying (AAGS). Readers can find more information about AAGS here. First, one of Brian’s slides stated that passive marks retain value and that improving data inclusion is both a benefit and a challenge. During his presentation, Brian stated that “passive marks are always going to have some level of value.” He also mentioned that “NGS has done a great job on improving the data submitting process.” NGS is developing models and tools for users to transition to the new NSRS. Tools such as OPUS Projects aim to help facilitate incorporating passive marks into the new, modernized NSRS.

(Image: NGS)

(Image: NGS)

Brian is not the only one who knows the importance of passive marks. As previously mentioned, the importance of saving surveying control marks was highlighted at the 2023 CLSA Geomatics Conference at Cal Poly Pomona/College of Engineering.

2023 CLSA Geomatics Conference. (Image: Cal Poly Pomona)

2023 CLSA Geomatics Conference. (Image: Cal Poly Pomona)

At the conference there was a panel session on saving survey monuments.  

The following is the abstract of the panel session: 

 “To discuss an ongoing problem of the destruction of land survey monuments and what the League of California Surveying Organizations (LCSO) and the California Land Surveyor’s Association are doing about it.” 

The panel consisted of five California County surveyors including David Farrell (LA Deputy County surveyor), Tom Herrin (San Bernardino County surveyor), Michael Lafontaine (Orange Deputy County surveyor), David McMillan (Riverside County surveyor), and Warren Smith (Tuolumne County surveyor). The presentation included discussion of a Monument Preservation Brochure and a Monument Preservation Guide.

Surveyors, engineers, and GIS professionals realize that inaccurate measurements can lead to boundary disputes, errors in construction projects, and environmental impacts. Passive marks are useful for validating measurements and spatial analysis. The panel noted that marks across California are in danger of being damaged or destroyed due to construction or insufficient public awareness. It was noted that addressing the loss of passive marks is important for maintaining California’s geographic information systems and preventing/averting legal disputes.

A goal of the guide is to provide individuals that oversee engineering work with strategies to save passive marks that are important to land boundaries and geospatial data.

The panel realized that survey monument preservation requires a collaborative effort from various stakeholders. By incorporating outreach, accurate locating techniques, efficient reporting systems and meticulous replacement strategies, California can safeguard its survey monuments for current and future generations.

The proposed guide will address the problem and outline a solution. The problem section would include topics such as lack of awareness, inadequate reporting, lack of funding, and the responsibility of the community. I have been informed that the guide will be posted here soon.

The panel members understood that the solution starts with outreach efforts and their draft guide lists the following potential outreach activities:

  • Educational campaigns at local schools, community centers, and public events to introduce the importance of survey monuments and their role in land ownership, land surveying and mapping.  
  • Community workshops with local civic organizations, homeowners’ associations (HOAs), and chambers of commerce to conduct workshops focused on survey monument preservation. 
  • Public awareness materials such as informational brochures, posters, and online resources that explain the significance of survey monuments and the potential consequences of their damage or loss can be distributed through public libraries, city halls, and online platforms. 
  • Media engagement with local media outlets can elevate public awareness and reinforce the importance of preserving these markers. 

As part of the public awareness material the group has prepared a draft brochure. They mentioned that having a website, such as the CLSA website, be a hub for contacts and information for research may help support the “save the mark” campaign. They included that GIS web maps will be a common place to find monuments and survey control.

The draft brochure states, “destruction of survey monuments within the public rights-of-way, mainly as the result of public works projects and private developments permitted by public agencies, is increasing, due to a lack of oversight and education concerning the importance of these monuments.”

The draft brochure addresses the following questions:

  • What Are Survey Monuments, Bench Marks & Geodetic Control?  
  • Why Are They Important?  
  • What Can You Do to Preserve Survey Monuments?  
  • Who Is Responsible? 
  • How Many Survey Monuments Are Really Needed? 

The use of passive marks is well known to land surveyors since they use these marks in their daily operations as described in the statement above. It is crucial to be informed of the importance of passive marks and what they can do to help preserve them. Any professional involved in urban development can have a role in saving passive marks from destruction.

The draft brochure outlines the following actionable steps that others can take: show all existing land survey monuments on improvement plans, grading plans, site plans, etc.; educate engineers, surveyors, plan checkers, inspectors, GIS professionals, and the public about the importance of monuments and the requirements to preserve them; prior to filing notice of completion for any project, have a licensed land surveyor validate that the monuments are in place; and request acknowledgement via a written statement in the permit process, that a licensed land surveyor has performed a field inspection and that no monuments are subject to destruction within the scope of the project, or that existing monuments have been referenced and perpetuated per Business and Professions Code §8771.

The section “How Many Survey Monuments Are Really Needed?” is kept simple and straightforward: all of them! These monuments are set to allow for the retracement (or to mark the location) of features and legal rights on Earth’s surface. 

There are many scientists who believe that active control stations are the solution to the surveying and mapping community’s positioning requirements.  

I believe active control stations such as NOAA CORS Network (NCN) that NGS promulgates are extremely important to the development and implementation of the NSRS. In the new, modernized NSRS access to the geometric component of the NSRS will effectively be defined by CORS and their coordinate functions. That said, this does not diminish the importance and requirement for maintaining and updating the coordinates of passive marks. 

The brochure is still a draft document and was not ready for publication at the time of this newsletter, but I have been told that it will be sent to all California county surveyors with instructions on what the goal is. Also, I have been informed that, as soon as it is publicly available, it will be placed on the websites of the California Land Surveyors Association and The League of California Surveying Organizations. 

I am encouraged by what California surveyors are doing to highlight the importance of passive marks. I would be interested in hearing from others on what they are doing to save passive marks or their thoughts on the importance of passive marks. Please feel free to email me at 

On a different topic, I would like to highlight that the NGS has announced the recipients of the NOAA FY 23 Geospatial Modeling Competition Awards. NGS awarded $4 million in grant funding to four institutions — Oregon State University, Scripps Institute of Oceanography, Michigan State University, and the Ohio State University — for projects that will research emerging problems in the field of geodesy, develop tools and models to advance the modernization of the NSRS, and help address a nationwide deficiency of geodesists.   

This is great news for the advancement of geodesy. I will address this in more detail in my November column. 

About the Author: David B. Zilkoski

David B. Zilkoski has worked in the fields of geodesy and surveying for more than 40 years. He was employed by National Geodetic Survey (NGS) from 1974 to 2009. He served as NGS director from October 2005 to January 2009. During his career with NGS, he conducted applied GPS research to evaluate and develop guidelines for using new technology to generate geospatial products. Based on instrument testing, he developed and verified new specifications and procedures to estimate classically derived, as well as GPS-derived, orthometric heights. Now retired from government service, as a consultant he provides technical guidance on GNSS surveys; computes crustal movement rates using GPS and leveling data; and leads training sessions on guidelines for estimating GPS-derived heights, procedures for performing leveling network adjustments, the use of ArcGIS for analyses of adjustment data and results, and the proper procedures to follow when estimating crustal movement rates using geodetic leveling data. Contact him at