Kent Washington and the Washington State Reference Network

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The City of Kent has expanded from a small farm community on the Green River to a city of more than 80,000 that relies on aerospace, warehousing, and light to medium manufacturing as sources of revenue. Kent is one of the last communities in the Greater Seattle area capable of providing low to moderate income housing.

Our survey group is asked to supply topographies, boundaries, right-of-way determinations, construction staking, asbuilts, and many other services for the City’s capital improvement projects. Several $20 million to $100 million CIP’s are constructed annually. We also supply horizontal and vertical control, and historical survey data to the private sector.

Today, all or portions of 43 sections fall within the survey group’s jurisdiction. Proposed annexations may add an additional 3000 acres. In the 1960s, Kent had roughly 10 square miles within the city limits and two three-man survey crews. Today, we oversee nearly 40 square miles. We still have only two crews but we’re down to two members on each now.

In January 2003 the city passed an ordinance requiring all development to be on the WSCS (83/91) horizontal and NAVD 88 vertical datums. In the Fall of 2002 and 2003, the survey unit ran differential levels for vertical control to approximately 70% of the city. Our records contain minimal pre-1960s survey information. It is unclear how or why the hodgepodge of concrete monuments, surface disks, tacks in lead, and sundry other types of control came to be in their present locations. The only reliable WSCS (83/91) values available in 2002 were those points published in the King County Control Survey (KCCS), conducted in the 1990s, or by the Washington State Department of Transportation (WSDOT).

Conventional on the ground/total station techniques were not a practical solution to expanding the City’s control network. GPS would allow us to stay abreast of our work load and expand our control network in a timely manner so the private sector and we could comply with our ordinance.

It wasn’t difficult to sell GPS to our superiors, but Kent was in the midst of layoffs and hardcore belt tightening so funding was extremely limited. In February 2003, we acquired three new Trimble 4600 single frequency GPS units, dual frequency receivers being beyond the scope of our minimal budget. They became our workhorses for establishing accurate horizontal control statically citywide.

Constructing the Network
Initially, our challenge was to determine a rational approach to constructing a primary city network that would solve our immediate problems and yet be flexible enough to remain viable in the future.

Since we were limited by the single frequency capabilities of the 4600, we decided that we would not attempt to create high accuracy reference network (HARN) quality points. If we could develop several good quality points as our primary network, within say 2-3 cm horizontal accuracy initially, then we could refine them as time went on and build off them to create other points throughout the city.

Puget Sound has several continuous operating reference stations (CORS) facilities scattered about. WSDOT has developed numerous high accuracy control points along Interstate 5 on the west hill, Highway 167 along the valley floor, and Highway 18 east of the City limits (Figure 1). The locations are perfectly suited to meet our requirements. Additionally, we have King County’s work as a check along portions of the East and West Hill districts.

As an initial test, we decided to create a central control point on the top of the parking garage next to our office. It is about 35 feet off the ground and is not masked significantly by adjoining obstructions. Our thought was to perform numerous sessions using the three 4600 units simultaneously, one on the garage and the others occupying known WSDOT and King County points, and then post process the data using CORS information to strengthen the geometry of our sessions.

No Formal Training
We needed to familiarize ourselves with the process and to learn the post processing software. We selected a section that has all of the primary section corners and quarter corners in place and that had been fully surveyed by King County GPS crews. Looking back, we unknowingly had a lot of beginners’ luck. Our results were almost exactly the same as King County’s published values, varying from 0 to about 0.07 feet. There wasn’t much formal mission planning in the beginning. It was more like, "Hey, great weather. Our work load is light this afternoon. Let’s go do a GPS survey." It seemed simple enough to gather the vector information and download it into Trimble’s TGO processing software. The processed results built a sense of confidence in us that would carry us through periods when harsh reality would throw some pretty big roadblocks on our rosy path. Formal GPS training was not an option for any of us at the time because of budget constraints.

Over the next few weeks we performed numerous static and fast static sessions using WSDOT monuments and, at first, CORS data. It became obvious the systems were on two distinctly different adjustments. The results we achieved using only WSDOT primary control fit very well and were repeatable on a regular basis. Since the state highway system affects so much of the city, we chose to hold WSDOT’s data as the source of our primary control network and process data using their reference Geoid. We had our garage station well established within a few weeks of making this decision.

Tying out PLSS corners in areas of future city construction or extensive private development was the next step. Much of this work was focused in areas where King County had conducted their GPS surveys. This provided us with the opportunity to develop our procedures and have reliable check points at the same time. King County’s control has been published for several years and local surveyors rely on it much as they had with its predecessor, KCAS. If we fit King County within reasonable limits, we elected not to publish alternative values. We also began to establish a network of secure points, similar to the garage point, citywide.

The city had purchased a Trimble 4800 transmitter/receiver for the operations division to perform conventional RTK for infrastructure inventory many years earlier. It was used sporadically and poorly maintained. We felt that, with some repairs, we could utilize it for RTK surveying, using the new network of secure points for base station locations.

Range was limited to about a mile and a half, so a network of 10 or 11 secure points was required to cover the city. Connectors, wires and batteries were always malfunctioning. We were pleased with the results when it worked, but neglected components cost more to repair than the equipment was worth and we were spending far more time repairing the gear than gathering data (Figure 2).

Our control network was not coming together as quickly as we had hoped. The city was coming out of its economic downturn and design surveys and construction work were preventing us from conducting the extended multiple static and fast static sessions we needed to expand the network. Furthermore, traffic control plans and flaggers were now required for any activity taking place in a city street for 45 minutes or longer. Eighteen-wheelers doing 55 to 60 mph push a lot of wind around and a couple of our tripods blew over with minor damage to the receivers. We built iron weights that attached to the tripod legs with Velcro, which helped stabilize the equipment but they added several minutes to any set up. Our safety concerns dictated that we develop a faster method of acquiring data
. It would be safer if we could reduce occupation times and use a two-man team consisting of an operator and a safety observer. We began hearing about the Washington State Reference Network (WSRN) in 2004. It could be the solution for what was becoming a serious problem.

Establishing the Framework
WS RN is a fairly simple idea with complex technical issues. Basically, multiple permanent fixed GPS receivers transmit data to a central computer. The data is processed and transmitted to a compact, lightweight remote roving unit via wireless telephone. The roving unit then utilizes this information to perform RTK surveys with exceptional accuracy. Outside of the federal and state governments, there are few, if any, individual agencies with sufficient physical or financial resources to independently build and maintain this kind of project. Recognizing the extensive benefits such a tool could provide, a consortium of users, spearheaded by the City of Seattle, was created and the basic framework of computers and receivers was established.

Budget constraints are always an issue in Kent so upgrades are usually hardfought battles over several budget terms. After some research and a few meetings with the WSRN folks, we realized what a great tool this could be. They supplied us with a free test account to inspect the program, which was the in the early stage of development. We used WSRN RINEX files to supplement the data acquired during static control sessions on various projects that we had underway at the time. Once processed, the results were excellent and we determined that this should be an outstanding solution to at least some of our problems. Furthermore, a single crew could be productive. There no longer was a need for three units to be moving around to different locations during a single session, as had been the case with the 4600s. Our challenge was to sell the idea of network RTK to our superiors, then hope that they could produce the monies necessary for us to participate in this unique venture. We built a PowerPoint presentation that showed the benefits to be derived and convinced our bosses that this was a tremendous asset.

With some creative financing, we were authorized to proceed, join the WSRN, and acquire two roving units and essential support hard and software. We purchased two Trimble R8 receivers, data collectors, and dedicated telephones in January 2005. The WSRN was still in beta mode at the time. We had some issues with connectivity and the Bluetooth telephone technology. We still do to a lesser degree today. For the most part though, the system worked well.

School of Trial & Error
Our schooling was pretty much by trial and error in a number of aspects. The system depends on several independent and complex systems working in harmony. When one or more does not work as intended, it can be a frustrating exercise to determine what, or who, the culprit really is. Satellite availability, PDOP, telephones affected by solar activity, operator error, interpreting data and many other gremlins may all play a part.

It seemed that everyone we talked to had a different way of processing data and interpreting results. What worked for one group never seemed to work for another. Survey chat rooms have dozens of all-knowing professionals, but the technology was so new that we rarely got useful advice from those sources. We learned that even the way the units were set up by the supplier the day we received them affected how to initialize them. To this day we have a slightly different procedure for one unit than the other. We had a period of several days where we were unable to connect to the system. We won’t embarrass anyone as to why that happened, but it wasn’t the sun spots we originally suspected. Our mantra for all future unexplained phenomena became "Well, you know, it must be sun spots." We learned that telephone companies can "improve" the way they provide service and, if they don’t let you know the secret handshake, your equipment will not respond appropriately. We also learned that the ephemeris changes over time and that it really helps to refresh it often. The instructions for the equipment are sometimes confusing or don’t address a situation at all, so it really helps to have a good sense of humor when some of these issues come up. As with so many tools today, it helps to thoroughly understand the limitations of the system and stay within those parameters.

We finally have become proficient to the point that the WSRN is a dependable tool in our arsenal. There are still times that we don’t or can’t communicate but they are becoming fewer, and we are learning how to identify problems quickly and adapt our methods to the situation with minimal lost time. Our productivity is up tremendously from where we were just a few months ago. The WSRN and R8s are currently used on just about every project with the exception of those that need precise leveling.

Currently, our procedure is to initialize a receiver over a known point then proceed to those points we are interested in. Once readings are collected, the equipment is returned to a known point for a position check and verification that all systems have functioned correctly. The check/recheck step is critical to the process. We discovered that, in a few cases, the information we accumulated was not correct. The magnitude of the errors have been from a few tenths to several feet. This happens often enough to make the check/recheck process mandatory.

Also, multiple sessions are conducted for redundancy. We currently occupy a point for four to six three-minute sessions over the course of a couple of days and average the results. If we are verifying a King County or WSDOT point, we will occupy the point at least twice. If the results are similar to the published values, then we accept them at face value. We use the term "on the marble". If the error ellipse is about the size of a child’s toy marble then we consider it to be within acceptable limits. All existing PLSS corners should be surveyed by the end of 2005. Once all PLSS corners within the city are surveyed, we intend to densify the network with four or five points strategically placed in each section (Figure 3).

Route surveys are accomplished by a combination of GPS and conventional surveying in many cases (Figure 4). All end points and certain intermediate points are surveyed by GPS, then conventional traverses are conducted between the points as required. Since the combined factor for Kent is so nearly 1.00, ground and grid distance are virtually identical for distances less than a half-mile. Points that cannot be occupied directly are tied out conventionally from two other points that are accessible to GPS coverage.

Vertically, we still have some discrepancies of as much as 0.2 feet from our differential levels. This has more to do with determining an accurate geoid for the city than inaccuracies in the city bench mark system. We are in the process of creating a custom geoid to reflect what is happening geographically across the city. Once that is accomplished, we should be able to establish vertical components reliably (Figure 5).

We have also used the equipment to establish horizontal survey control on construction and topographic projects. However, we have yet to do any significant construction staking since we are still uncomfortable with the elevation results we are obtaining. We propose to use it next Spring on the grade and fill portion of our South 228th Street Extension from the valley up to I-5. It seems unlikely that anything will replace differential levels on the valley floor where designs have slopes as flat as billiard tables. We have ample bench marks in most areas to make differential levels easy and quick.

It is our intent to have a beta version of an interactive web site available within a year giving everyone Internet access to our survey control database. We currently have an interactive map and data base available to us internally. When we r
eceive a call we can pull the map up on our computers, zoom in to the subject area, acquire data and transmit it via email to the requesting party. Data acquisition can be completed and in the caller’s hands in less than 5 minutes (Figure 6). We have recorded a few Records of Survey to date. Many more will be recorded in the future.

We feel that this is one of the best ways to let the private sector know how we are approaching boundary and control issues and document the monuments that are set in conjunction with construction. It is our intention to record a survey that documents the control network we employed and developed originally. RCW 58.09.070 requires a map depicting the control scheme used if Washington State coordinates are shown on a survey. Rather than duplicating that map on each survey, we feel that referencing this recorded document should keep us, or anyone utilizing our data, in compliance.

Purists might scoff at our system. We aren’t Blue Book material yet but we have a dependable network that meets our present needs and that will be refined over time. We were on the right track for the results we wanted before the WSRN. But the WSRN has allowed us to get closer to our destination a lot faster and have many more alternatives available once we arrive. We are about one year ahead of the timetable we originally proposed in 2003 and gaining additional days every time the gear comes out of the truck. We consider it one of the best investments the City of Kent has made. Now if we could just get rid of those pesky sun spots.

Author note: Special thanks go to Gavin Schrock and the Seattle Public Utilities/ Engineering Department for their commitment and total dedication to developing and perfecting this extraordinary tool for the surveying community.

Steve Merryman, a licensed surveyor for more than 26 years, is the Land Survey Supervisor for the City of Kent, Washington.

A 1.863Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE