• Dale Hill

Salton Sea SMASH

Updated: Jun 19

Recovery Date: May 28, 2022

Total Marks Recovered: 47

Discs Recovered: 44

Non-Disc Type Recovered: 3 (pipe cap: 1, Flange Encased Rod: 2)

NOTE: This trip covered a lot of ground and I recovered 47 survey marks today; not only are there a lot of photos and the accompanying narrative, but I've also provided some insight into how I prepare for a trip. 😊

Road Trip!

Weekends are usually a coin toss for me; go for a long hike? or head out on a Survey MArk Scavenger Hunt? Since I hiked to San Bernardino Peak last weekend, I opted for a SMASH today. Dedicating a whole day to looking for survey marks requires advanced planning, and over the previous few weeks, I had been researching survey marks in the vicinity of the Salton Sea. Today's adventure is a 250-mile road trip to and around the 15 x 35 mile Salton Sea, looking for survey marks as I go.

Last December, I made a trip to Obsidian Butte, located at the southeastern end of the Salton Sea, to find the survey marks at the Butte and explore the area. I recovered over 30 survey marks on the drive there, mainly along SR 78, which made the few marks I found at the Butte seem more like a footnote to a full day of recoveries than the trip's focus. However, I became fascinated by the region's history as I completed my post-trip research. See my article Obsidian Butte SMASH for more details about that adventure.

Getting Started - A Worthwhile Tangent on Preparation

Back to my research, I typically use various online tools published by the National Geodetic Survey (NGS), the Bureau of Land Management (BLM), and local County Surveyor Offices when planning a SMASH.

Thomas Jefferson authored a Resolution that would become the basis for the official mapping of our country. The Land Ordinance of 1784 divided the land west of the Appalachian Mountains, north of the Ohio River, and east of the Mississippi River into separate states. Updates in 1785 created the Public Land Survey System (PLSS), the basis for the grid system that divides the country into 36-square mile Townships (further divided into 36 1-square mile sections). The PLSS also provided the first mechanism for funding public education and designated the land in Section 16 of each Township to construct a school. Today, you can still find public schools in Section 16 of their respective Townships, but I digress. Most of the "heavy lifting" of mapping the United States began in the early 1800s and the predecessors to today's NGS were responsible for that work.

The National Geodetic Survey (NGS), formerly the United States Survey of the Coast (1807–1836), United States Coast Survey (1836–1878), and United States Coast and Geodetic Survey (USC&GS) (1878–1970), is a United States federal agency that defines and manages a national coordinate system, providing the foundation for transportation and communication; mapping and charting; and a large number of applications of science and engineering. Since its foundation in its present form in 1970, it has been part of the National Oceanic and Atmospheric Administration (NOAA), of the United States Department of Commerce.

NGS Data Explorer Map - Info and Tutorial

The tool I rely on the most is the traditional NGS Data Explorer Map; it allows me to search an area for survey marks that have been assigned a Permanent ID (PID) and are cataloged by the NGS. Hundreds of thousands of survey marks across the country are in the NGS database, these represent only a fraction of ALL survey marks, but it's an excellent place to start. As you can see from the images below, the more you zoom in on an area, the more detail you get.

In the first image, I centered my search on the middle of the Salton Sea (the red "X") and had my criteria set to find all marks within a 25-mile radius. The colored squares indicate the density of survey marks in that area, the green squares mean less than ten survey marks, and the orange represents a cluster of 10 to 99 marks (the number in the box tells you how many are in that cluster). Survey marks are often collectively referred to as "benchmarks," but a "true" benchmark is the type of Geodetic Control that measures elevation.

As you zoom in on a particular area, the view changes to the icons that represent different types of markers (or Controls); clicking on a control symbol opens a small information pop-up that has the essential details about the mark and a link to the full datasheet on the NGS website. The datasheets provide all the known geodetic control information about a particular mark, including its location and how to reach it (often referred to as the "To Reach" description). I usually print the datasheets for each survey mark I am looking for so I can refer to them in the field; alternatively, I copy and paste pertinent notes about each mark into the waypoint file that I save on my GAIA GPS for convenience.

In the third image above, I highlighted the Station: BOMBAY (PID: DW9086) (click on the link to see the full datasheet.) Deciphering all the information on the datasheet can be a chore; however, the NGS has a nifty PDF guide for understanding how datasheets are formatted, a beneficial resource in my book. They also have an hour-long webinar about datasheets which you can watch online or download.

Transfering Survey Mark Locations to Waypoints

Researching and setting waypoints is a tedious process but worth the effort, especially in areas with limited or no internet connection. Using the coordinates and other information from the datasheet, I create waypoints in my GAIA GPS App (desktop) for the marks on my target list. While not all the coordinates listed in the NGS database are 100% accurate, they will get you close to the mark. I have recovered survey marks that were as much as 300 feet from the scaled coordinates listed on the datasheet (this is where it pays to study the "To Reach" descriptions as they can be very accurate). Once I have the marks in my GAIA, I can download my maps for local use when I don't have a signal.

I checked T-Mobile's coverage map for the area, and it appeared I would have good signal strength all around the Salton Sea. Consequently, I didn't go crazy adding waypoints for all the marks I had on my radar, plotting only a few critical recoveries, and relying on the interactive maps for the rest.

Interactive Maps and Active GPS Searching

The NGS Data Explorer is a static tool; the NGS does have an interactive map they call the Beta NGS Map, which is very cool because it shows you nearby survey marks based on your GPS position; you can then tap on the icon to view more information, including a link to the datasheet. The active hyperlink is awesome IF you have a reliable internet connection!

Let the Journey Begin!

My primary search area was on the eastern side of the Salton Sea (getting there is half the effort); it is just about 80 miles from my front door to the intersection of S 22 (the Borrego Salton Sea Way) and SR 86. From there, I needed to go 25 miles north to Mecca, then cut over to SR 111 and head south.

I have traveled the route through Borrego Springs on the S 22 many times to reach trailheads for hikes and search for survey marks, so I didn't expect to stop and look for additional survey marks on this part of the drive today. However, I've learned that no matter how thoroughly I think I've searched an area, there always seems to be another mark to find; today was no different. Today I made three recoveries near the S 22 that I missed previously. A PLSS Section Corner and two San Diego County Engineer Department discs. A nice bonus!  👍🏻

When I initially searched for the survey mark AD 31 last December, I didn't have its exact coordinates and relied solely on my USGS Topo map to get me close. Once in the general vicinity of the mark (based on the symbols on the topo map), I scanned the area for a witness stake or any other sign that I was in the right place. Still, the area was a roughly graded berm, and I presumed the grading had destroyed the mark.

As it turned out, the mark was buried, and the original witness paddle had been dislodged and was nowhere near its original position. However, since my first attempt to locate it, someone had recovered and staked it up!

When I first looked for AD 30, I was searching close to the edge of the road, a distance consistent with other marks I had recovered in the area. Today, I decided to check around the perimeter of the fence enclosing a communications tower; as it turned out, the mark was 130 feet from the edge of the road, mostly buried and under a bush not far from the fence! 🤷🏻‍♂️

Northbound on SR 86 - Successes and Failures


I had dropped several pins for marks that were in the SR 86 corridor; I was able to pull off the road and locate the US Army Corps of Engineers (USACE) Triangulation Station CONN and its related Reference Marks quickly. The California Division of Highways (CDH) Station DYKE and the Riverside County Surveyor mark DESERT SHORES were accessible via local surface streets. I even had a surprise Proximity Recovery of a CDH post-mile marker while making the DYKE recovery! The DESERT SHORES mark was in the northwest corner of the Desert Shores Community Park. Very happy with seven quick recoveries, I celebrated with a mid-morning snack break at the park before moving on to my next set of marks. 😉


Sometime last year, I had set pins for ten survey marks on the western side of SR 86 near Travertine Rock; they included regular Benchmarks, a couple of section corners, a post-mile marker, and of course, the mark at the top of Travertine Rock. These marks weren't too far from Desert Shores Park, so they were my logical "next choice." So I pulled off the 86 and turned left onto the sandy frontage road leading to the base of the rocks. The road seemed well worn, and I wasn't too worried about the road condition until I reached the rocks and pulled off the road to park.

After getting stuck in soft sand, I passed on looking for these marks!

As soon as I committed to pulling over, I could sense that the sand was a lot softer, and I immediately tried to back up onto the frontage road, but my tires just spun out. I tried to rock forward and back gently, but I wasn't going anywhere. I got out to assess the situation and figured I could dig out my wheels and use some heavy cardboard that I had in the back of the truck to gain purchase. I only needed to go back about 10 feet, and since I'd managed to get out of similar situations, I wasn't overly worried.

I had been working at it for about 15 minutes when a full-size 4 x 4 truck pulled up alongside where I was stuck. The two guys had seen me from the road and offered their help; thanking them, I grabbed my tow strap from the truck, hooked it up, and they pulled me back onto the more stable dirt road surface; from there, I would be able to back down the road to a point where I could safely turn around.

After getting the truck squared away, I was irritated that I had gotten stuck and was rapidly losing interest in these marks. I did climb the smaller rock pile looking for G 521, but my heart wasn't in it, and when I didn't find it right away, I mentally checked out on these targets.

Based on the graffiti and amount of trash in the area, it seemed to be a party spot; I rationalized that any mark that may have been there could have long since been vandalized. I returned to the truck, backed down the frontage road, and headed towards Mecca and SR 111. For the time being, those will remain as "stars" on my map, indicating potential recoveries.

Mecca to Niland and (Almost) Everything In Between

Headed northbound on SR 86, I took the exit for Mecca and went east on 66th Street; this becomes a short connector to Grapefruit Boulevard, better known as SR 111. In Mecca on southbound SR 111, this SMASH got underway in earnest.

I had selectively dropped pins (waypoints) on my GAIA map for various survey marks on this route between Mecca and Niland, 46 miles down the road, but as I had mentioned earlier, I had counted on using the interactive NGS Beta Map as my primary search tool. Unfortunately, however, I lost signal once I drove through Mecca and wouldn't get it back until I was headed westbound on SR 78, pointed towards home!

Without the "easy" tools, I had to work for these recoveries 🤣 I used my USGS Topo map as a reference to find many benchmarks; the map uses the capital letters "BM" and "X" plus the elevation for the mark to show where the benchmark is located. So just having the USGS Topo map layer active got me close to those marks, even without exact coordinates (this is also how I locate "Drive-By Recoveries"). A benefit of using the USGS Topo is that it will also identify section corners; as I headed down SR 111, I noticed a potential corner between the tracks and the road, an easy recovery!

Locating the NE Corner of Section 20, Township 7S, Range 9E, SBBM using the PLSS/USGS overlays

The thrill of the hunt is using my knowledge of how and where survey marks are typically placed to be able to find them without the interactive maps. A common setting (particularly along main roads) is in a concrete monument with a nearby witness stake.

There is a wide range of materials used for witness stakes (wood lathe strips, 4x4 posts, steel T-bar fence stakes, rebar, etc.). Still, the one thing they have in common is that they're usually standing out by themselves and look somewhat out of place (other than the fact they are signaling a survey mark is nearby.) Also, as you can see from the Section Corner below, sometimes there is more than one type of witness stake, making a recovery much easier!

Two other common places to mount survey marks near roadways are bridge abutments and culvert headwalls, as they provide a reasonably stable mounting surface.


I found four survey marks mounted on bridges, the first one I just happened to spot as I drove over the bridge (I'm in the habit of scanning sidewalks, top rails, and abutments for signs of survey marks). When I checked out the disc, I found that it was a reset mark.

There are established procedures for handling survey marks that will be disturbed or destroyed during construction activities. During the pre-construction survey, surveyors will note any survey marks that will be affected by the construction and can reset the mark once the construction is complete. The reset could be in the original location (optimal) or somewhere nearby based on the nature of the construction. Resets are not always reported to the NGS, so it is not uncommon to find the datasheet for the original mark but no information about the reset. I also expect that many marks have been destroyed because they weren't identified in a pre-construction survey. 🤷🏻‍♂️

USBR BM 12 A RESET was set in the center of the top rail of the bridge over a drainage ditch. This was a reset of a Bureau of Reclamation disc that probably wasn't stamped with a date when it was first established.

In addition to the standard photos of the mark and its location, I've included a graphic above that I put together from the datasheet for the original mark, annotated to identify some of the more useful information to me when I am searching for marks. The DOT set this mark in 1992, likely when the road was widened, and the bridge was rebuilt. Notice the original setting was in the bridge curb about a foot above the highway (the "one foot" is obscured by the frame I use when I create my recovery graphics 🙄; it's in the lower right-hand corner, link to the full datasheet here), the reset is on the top rail of the bridge as shown in the second picture above.

The remaining three marks that I found on bridges were located much the same way; as I approached a bridge, I'd slow down (there was no traffic) and scan the top rails for any sign of a disc. To make matters easier, most of them had been painted to identify where the mark was.

During my research to learn more about these three survey marks, I found the CALTRANS Log of Bridges on State Highways. These bridges are in Riverside County, which falls under CALTRANS District 8 (San Diego and Imperial Counties are in District 11.) It was interesting to trace these bridges back to the log and decode all the information about them; here is a rundown of the data:

  • Postmile. This is the location of the bridge to the closest 100th of a mile, as measured from the start of the route. Postmiles are most commonly calculated from the origin of the route and increase south-to-north, or west-to-east, depending on the route. The numbering resets at county lines. There are only five routes in California that do not follow this rule; they are known as backward routes.

  • Bridge Number is the official structure number assigned by CALTRANS and consists of two digits followed by four, with an optional alpha character as a suffix. The first two digits identify the county (56 is Riverside), and the next four represent the bridge's unique number. The suffix identifies the function of the structure. There is no suffix in the three above, meaning the structures carry two-way traffic.

  • OU - this is simply Over/Under; the three above are all identified as "O" followed by the descriptive information of what they go over; for example, these go over named ditches.

  • Structure Type This is a 3-digit number; the first digit identifies the type of material (1=concrete), and the second & third characters identify the type of design (19=Culvert).

  • City. If the bridge is within city limits, this will be a 3-character code to identify the city.

  • Bridge Length. The length of the bridge from paving notch to paving notch (given in meters)

  • Width. Generally, the outside-to-outside width of the structure.

  • Number of Spans. The number of spans in the structure.

  • Min VC Over Roadway. This is the minimum vertical clearance over the traveled portion of the route (in meters) when the bridge goes over a roadway.

  • Sidewalk Lt. Rt. This is the width of the sidewalks on each side of the bridge, if applicable.

  • Year Built. Self-explanatory.

  • Year Widened/Extended. Self-explanatory.

  • Permit Rating. This is a string of five characters that correspond to a color-coded permit system for commercial vehicles that require a Motor Carrier Permit. The five characters provide the permitted capacity for different vehicle sizes: 5, 7, 9, 11, and 13 axle vehicles. These bridges above are rated "P" for all five vehicle sizes. (P=Purple permit capacity) the permitted capacity is determined by a table lookup based on the number of axles, the distance between the axles, and the weight of the vehicle.

  • Posting. A single alpha character that gives the status of the structure (open, closed, new, etc.)

Since all three of these bridges were widened or extended in 1997, it's a safe bet that the survey marks placed on them were set in 1997 as well. After recovering these few, I decided to pass on collecting more of them since they were all the same and didn't have any unique markings.

Much like the NGS Data Explorer, CALTRANS has a GIS Map of State Bridges where you can search an area, click on an icon, and see the detailed information for that specific bridge. I created this quick graphic to highlight the bridges around the Salton Sea and specifically highlight the Elm Ditch Bridge, which was one of my recoveries.

State bridges on the routes surrounding the Salton Sea

Unless you are a Bridge aficionado or your work involves bridges, you probably don't know (or care 🤣) about these resources (I sure didn't). However, I do find it fascinating to learn new stuff like this, no matter how mundane or irrelevant it may seem to others.


I found four survey marks mounted on culverts; the first one was on the headwall of a culvert that ran under the roadway and was one of the waypoints I had dropped based on my initial research. However, when I checked out the disc, it was another reset that wasn't updated in the NGS database. EE 516 RESET was located at the top of a culvert headwall, set by the California Department of Transportation in 1990, very likely when they widened the road.

The remaining three were found on Culverts that ran under the railroad tracks. Safety reminder: be careful around active railroad tracks; I made all of these recoveries between the many trains I saw during the day.

It was pretty cool to find a US Coast & Geodetic Survey (USCGS) Benchmark (BM) disc G 70 dated 1928; this benchmark disc design was first used in 1900 and continued through 1972. If you're wondering, the NGS no longer sets new benchmark discs. However, most of the older USCGS discs I've found in Southern California go back to the early to mid-'30s.

National Geodetic Survey

In addition to the Vertical Control Mark A 1256 that I found on a railway culvert (above), I found two other survey marks from the NGS, Z 1255 (PID: DW0566); another Vertical Control Mark, and DHLG A (PID: AH8516) a flange encased steel rod. DHLG A was the second steel rod marker that I found today! Z 1255 was set slightly below ground level, inside a pipe that was set in a concrete monument. I usually find these in a 6" PVC pipe (surrounded by concrete) with a screw cap, but this one was in a smaller diameter pipe that was significantly rusted (I cut my hand on it while trying to clean and chalk the mark!)

A BONUS Azimuth Mark!

If you follow my survey mark hunting adventures, you know that I get excited when I am able to locate an Azimuth Mark. These typically don't have precise coordinates posted; instead, they are usually referenced by a geodetic azimuth (bearing) from the station they point to. Additionally, they are at least a half-mile or more from their primary station. I spotted this one because it was mounted on a pipe sticking a couple of feet out of the ground and had a witness stake plus a witness sign! I originally thought it was a section corner but was pleasantly surprised to find it was the US Coast & Geodetic Survey disc: SALTON AZ MK (PID: AH851). I will have to put it on my list to go back and recover the Salton Station disc and its reference marks!

High Precision Geodetic Network (HPGN)

According to the NGS, an HPGN is a designation used for a statewide geodetic network upgrade. The generic acronym HARN (High Accuracy Reference Network) is now used for both HARN and HPGN and was adopted to remove the confusion arising from the use of the two acronyms. A HARN is a statewide or regional upgrade to the accuracy of NAD 83 coordinates using Global Positioning System (GPS) observations. Many of the survey marks used in this network are named HPGN stations.

Today I recovered two HPGN stations, HPGN D CA 11-GM (PID: AB5448), a densification station, and HPGN CA 11-11 (PID: DW9075). The original spacing for the HPGN was 40 miles (64 km). To improve access to the HPGN, densification monuments were set at 10-15 mile (16-24 km.) spacing, providing more control points and increasing network accuracy.

GLO CORNER 2: An Atypical PLSS Section Corner Recovery

PLSS Section Corners are usually set by the Bureau of Land Management or one of its predecessor agencies, so I was very surprised when I found this section corner and its reference marks set with standard US Coast and Geodetic Survey discs! I spotted the reference marks first as they were in obvious concrete monuments, each with a traditional witness sign nearby. The great thing about finding both reference marks is they triangulate on the station disc.

Following the arrows on the reference mark, I found the station disc (the actual corner) in a thicket of brush and small trees. As you can see from the photos, there were a number of witness paddles and a witness sign nearby, but the mark was not readily visible. I cleared much of the thorny brush out of the way and had to dig down about 6 inches before I hit the concrete monument. The name of this Triangulation Station is GLO CORNER 2, however, it designates the NE Corner of Section 15, Township 10S, Range 13E, SBBM.

US Coast & Geodetic Survey (USCGS)

Two other USCGS survey marks I found today (not already listed) were Z 612 (PID: DW0133) and NILAND RM 2 (PID: DW0195). Z 612 was an easy find because of the nearby witness sign, and the NILAND reference mark was only a few feet away from the HPGN CA 11-11 survey mark!

California DOT Named Stations

I have recovered many survey discs that I categorize as "named" stations (as opposed to numbered survey marks). These are California Department of Transportation discs, set in concrete monuments, with two reference marks nearby. The origin of the name may, or may not be, obvious. I found seven named stations today; CONN and DYKE are listed above. The remaining five are:

  • CREEK - PID: DW9089

  • BUREAU - PID: DW9087

  • BERTRAM 2 - PID: DW9083

  • WAGON - PID: DW9117

  • POPE 2 - PID: DW9111

It was easy to spot the concrete monuments and witness posts from the road since there was little vegetation to hide them. I haven't researched the names for all of these, but CREEK is obvious as it is near Salt Creek.



I noticed some of the witness paddles were stamped with the station name and distance to the survey mark, so I started adding photos of the witness paddle to my database.



The Final Two Recoveries - Irrigation Ditch Survey Marks

As the day wore on, I knew I had to draw the line somewhere and head home. After passing through Niland, I noticed a waypoint for the named station DONALD at the intersection of SR 111 and McDonald Road (my turn to head back west). This area is all farmland and irrigation ditches, so I figured this would be an easy recovery; I pulled over at the intersection and immediately spotted a disc on the wing wall for the irrigation channel.

Upon closer inspection, it wasn't the one I was looking for, but I'll take the unexpected proximity recovery! 😉 As I was prepping the disc HUDSON POWER RANCH No. 53, I noticed another disc on the opposite wing wall; this one was stamped IMPERIAL IRRIGATION DISTRICT but didn't have any other markings.

Excited to end my day on a "2-Fer", I finished documenting both recoveries and headed home, enjoying a beautiful sunset as I returned on SR 78 through Anza Borrego Desert State Park.


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