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  • Writer's pictureDale Hill

The 7-10 SMASH

Total Marks Recovered: 7

Discs Recovered: 5

Non-Disc Type Recovered: 2 (1 New)

New Agencies: 3

Notes: New Agencies

  • Metropolitan Transit Development Board

  • Sycuan Band of Kumeyaay Nation

  • Santa Fe Depot LLC

San Diego SMASH - Part II

I really didn't plan it this way, but today's Survey MArk Scavenger Hunt was a lot like picking up a 7-10 Split at the bowling alley 🎳 with my attempted recovery of the San Diego Zero Milestone splitting my day of successful recoveries right down the middle. Initially, I planned to write up all of today's recoveries in one article, but the more I researched the history of the San Diego Milestone and its removal last year, the more I felt it deserved its own article. You can read about what I found, and didn't find, in the article The Missing Milestone Monument.

While recovering the Zero Milestone was the focus of my day, it wasn't the only mark on my list. Many of my recovery outings start by researching the National Geodetic Survey's online database and the GPS on Bench Marks database for potential targets, but today I searched through the US Benchmark category on the website, looking for marks others have found in San Diego. The database is not the most current, but it's a good starting point, so I selected 15 different marks to search for and dropped pins in my GAIA GPS App to mark their expected locations, from there I planned my not-so-direct route downtown and I was off!

On the Trolley Line - A New Recovery

It still amazes me when I make new recoveries and I am always excited when I recover a new type or one issued by an agency that I haven't encountered before. My first two recoveries today fell in the latter category. Both marks were control points along the Trolley tracks, the first one was at the trolley station in La Mesa and the second, in Lemon Grove.

The San Diego Metropolitan Transit System (SDMTS or sometimes just MTS) is one of the oldest transit systems in Southern California, with predecessors dating back as early as the 1880s. These were marked MTDB for the Metropolitan Transit Development Board which was the most recent predecessor agency to the modern-day MTS. These were the first MTDB discs I've found 👍🏻

You'll notice that these are stamped with two numbers separated by a cross (+), as I learned with an earlier recovery these numbers represent the distance from a surveyed Station. A "station" is a segment equal to 100 feet and the offset is the distance from the last station. Station numbers usually increase from the beginning of the project to the end of the project. Also, stationing usually runs from South to North or from West to East.

Gaging Station - Another First!

This is my first Gaging Station mark, the gaging station is shown in the first picture below. The US Geological Survey has gaging stations at thousands of locations across the United States, these stations collect information and transmit it to the USGS via a satellite communication system, which they, in turn, make available to the public via their website.

These stations are used to automatically monitor streams, wells, lakes, canals, reservoirs, or other bodies of water. They collect a variety of information that is used for flood prediction, water management, recreation, and navigation purposes. There are many different types of gages and, now that I know to look for them, I will try to find some of the different types.

At these Gaging Stations, the U.S. Geological Survey (USGS) sets gages to read the stage above a specified reference surface, this reference point is called the gage datum. In general, a datum is a base elevation used as a reference from which to determine heights or depths.


Side Note on Datums

Okay, I'm not a scientist, geologist, surveyor, or anything like that, so I get to geek out a bit over the new stuff I'm learning. Professionals in any of those fields just know this stuff, but my head is swimming with new information I'm finding as I dive into figuring out just HOW these survey marks are used and all the stuff related to them.

Here's some basic information about three general datums that I found on the National Geodetic Survey (NGS) and National Oceanic and Atmospheric Administration (NOAA) websites:

  • Horizontal Datum: A specified coordinate system for a collection of positions on the surface of the earth. Traditionally, horizontal datums have used classical surveying methods (i.e. Measuring distances and angles through triangulation surveys) to best fit the surface of the earth

  • Vertical Datum: A surface of zero elevation to which heights of various points are referenced. Traditionally, vertical datums have used classical survey methods to measure height differences (i.e. geodetic leveling) to best fit the surface of the earth.

  • Tidal Datum: A standard elevation defined by a certain phase of the tide. Tidal datums are used as references to measure local water levels and should not be extended into areas having differing oceanographic characteristics without substantiating measurements. Tidal datum is collected through a network of stations (follow the hyperlink above to go to the stations)

New Datums. The NGS is currently working to update all the current datums and plans to publish the data in 2022. The GPS on Bench Marks initiative that I am participating in is a key part of collecting updated information for the new datums. The reason why the GPSonBM program is so important is that the new datums will rely primarily on Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS).

According to the NGS FAQ on the program, the current horizontal and vertical datums were defined primarily using terrestrial surveying techniques at passive geodetic survey marks (commonly the brass discs). This network of survey marks deteriorates over time (both through unchecked physical movement and simple removal), and resources are not available to maintain them


This leads me back to the disc...

In order to check and verify datums, they are referenced to fixed points known as benchmarks. The benchmarks are originally set as a result of a survey and their exact elevation and coordinates are recorded, they can then be used as a reliable control point in verifying the datum. Often, the term benchmark is used generically to identify ANY disc-shaped survey mark, but a true benchmark is a specific type of survey mark used to denote elevation.

Phew, All that to explain that I found a cool new disc stamped GAGING STATION 🤣 This one didn't have any specific stamping on it to identify its location or the date it was set. I have no idea (yet) if this is standard, or if I'll find one that has additional stamping, only time will tell.

In the Heart of Downtown

Shifting gears a bit to some non-disc marks. While it was the cool brass discs that I'd find at mountain summits that sparked my interest in survey marks, there are so many other types of survey marks out there. In all the marks that I have recovered, I have 42 different types, 16 of those types are different categories of discs, and 26 are "non-disc" types. Of the 16 different categories of discs, one is type "Z" which is a catchall type that usually requires further explanation in the details of the datasheet. The non-disc types can be natural objects like trees, or manmade structures like towers, domes, or crosses.

Today I added a couple more manmade structures to my database with the southwest corner of the Grant Hotel and the west tower of the Santa Fe Depot. The SW corner of the Grant Hotel is type Z where the mark at the Santa Fe Depot is type 55 = Tower. I didn't take a close-up picture of the corner of the Grant Hotel where it meets the sidewalk (although the next time I'm downtown, I'll do that).


I've had more than one person ask me "how can they use the corner of a building as a survey point?" Well, going back to the datasheet and checking out the description of the survey mark when it was first established in 1908 I found the following:

"The point used was the SW Corner of the U.S. Grant Hotel at San Diego. The corner is the intersection of the D and Third Street faces of the main part of the building, excluding cornices and projections. The building is in the process of erection and upper part is not yet completed (Feb. 1908)" [sic]

A couple of updates over the years, D Street is now called Broadway and an Eccentric station was added in 1923 and was marked by a chiseled cross (+) in the flagstone at the top of the SW corner of the hotel.

Additionally, back in the 1930s, there were two steel-latticed radio towers mounted on the roof of the U.S. Grant Hotel, and each was designated as a survey mark. DC1718: SAN DIEGO GRANT HOTEL W TOWER and DC1726: SAN DIEGO GRANT HOTEL E TOWER. However, neither tower remains and they are thought to have been removed as part of a major renovation in 1984. I found this photograph in the California State Library digital photo archive (item #2) that shows the towers in place.

U.S. Grant Hotel [c. 1936] From the California State Library Photo Archive

The U.S. Grant Hotel was opened in 1910 by the son of President Ulysses S. Grant, it has undergone many changes of ownership since falling into disrepair in the 1970s. In 2003, the Sycuan Tribal Development Corporation (Sycuan Band of Kumeyaay Nation) purchased the hotel. The tribe said it had a special connection to the hotel because it was President Grant who signed an 1875 executive order that set aside land for the Kuymeyaay, including the Sycuan reservation in East County. It's also interesting to note that the exact land the building is built on was originally owned by the Kumeyaay for thousands of years. IMHO, I believe the real reason they purchased it was that it was on their native land, not so much that President Grant "set aside" reservation land for them in 1875. Don't get me wrong, I'm sure they did appreciate that, but reclaiming tribal lands 10,000 years old would seem more important to me. 🤷🏻‍♂️The hotel is currently operated by Marriott Hotels & Resorts as a part of its Luxury Collection.


The Santa Fe Depot (as it was originally designated) officially opened on March 8, 1915, to accommodate visitors to the Panama-California Exposition. Hosting the 1915 Exposition was a major influence behind much of the development of San Diego at the time, including the evolution of Balboa Park into one of the most unique cultural parks in the country. (I had to sneak in a reference to Balboa Park! I operated my pedicab business weekends in the Park for 11 years, providing shuttle service and offering historical tours on my pedicab, so I learned a LOT about the Park's history 😁).

The Depot was built by the Atchison, Topeka, and Santa Fe Railway in a Spanish Colonial Revival style. This architectural style, and particularly the signature twin domes, was prevalent throughout San Diego at the time. (The Spanish Colonial Revival architecture is one of my favorite aspects of Balboa Park!) The Depot is listed on the National Register of Historic Places and is still a very active transportation center in San Diego. It also houses the downtown branch of the San Diego Contemporary Art Museum.

Back to the Embarcadero

I wrapped up today's recoveries with two more San Diego Unified Port District GPS Control points. While researching tidal bench marks, I recently learned that some of these SDUPD marks are also assigned as tidal benchmarks and have a unique Vertical Mark Number (VM#) but I've yet to find one of those. The first SDUPD disc I found was SDUPD-013 and it was by a traffic island on West Market Street downtown, I happened to see it as I was crossing the street. I found that one before I began chalking up the marks to make the stamping more obvious, I'll probably revisit that mark just to chalk it and update my photo in my database 😁

Wrapping it up...

I always have fun when I'm out searching for survey marks, even when I don't find all the ones on my list. As much fun as I have in the field, I am simply blown away by the "back office" part of this hobby, researching the background and history of the marks so I can enter them into my database and write somewhat coherently about them in articles such as this. It seems like every mark has SOME story attached to it.

As I mentioned above, I separated today's trip (July 10th...hence the 7-10 title) into two separate articles, the first focused just on the Zero Milestone Monument and of course, this one dealt with everything else I found today. If you made it all the way down here to read this, I thank you from the bottom of my heart. For me this hobby is more than going out and collecting's the opportunity to learn new stuff, and learning new stuff is never a bad thing 😉

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