astrodanco

Heck, I have an old Garmin GPS 45 that still works. I found it while cleaning out my garage. It shows the wrong date, but otherwise works fine.

And now thanks to men like Michelson if we want to know the distance we bounce a laser beam back and forth and measure the round trip travel time.

Yup, the city where I live is stuck in NGVD29.

I hope you are aware that there are different types of benchmarks. The two major divisions are vertical control vs. horizontal control. Historically two separate groups did horizontal vs. vertical controls and the two separate groups did not interact. For horizontal control the position was important, but the elevation was not. For vertical control the elevation was important, but the position was not. Positions of horizontal control were precise, but elevations not. Elevations for vertical control were precise, but positions not. Many positions for vertical control were obtained by reading the description to determine where that description placed the mark on a USGS topo map, then measuring the scaled position off the topo map with a ruler. That's what makes benchmark hunting so fun. The vertical controls were precisely leveled through and the horizontal ones were not. The position of a mark is not punched onto the mark, but shown on the datasheet and can be corrected. Elevations can change due to things like subsidence. Old vertical datums can become obsolete. Positions can change due to things like plate tectonics. Horizontal datums can differ significantly. If the datasheet isn't accurate, any surveyor can spend 4+ hours collecting GPS L1+L2 observations on the mark and submitting it to NGS via OPUS shared solutions. OPUS PROJECTS will be used for this purpose in the near future. Post 2022 or thereabouts I suspect very few benchmarks will warrant preservation as the NSRS will become active rather than passive. We're quickly reaching a time when most passive benchmarks will be a quaint legacy. Bill93 will be along shortly to correct me.

I hope she's not assuming the power squadron is a more reliable source than you are because they're not. Did you submit photos of both the monument and of its locale?

The other observations can be from CORS or PBO or what have you sites. They don't have to be your own. I would have a hard time explaining it, but it lets you arrange all the stations and how they are used by OPUS during processing into in a hub and spoke arrangement that improves your statistics. It's a better way of processing on their end though move involved to set up and perform on our end. See ftp://geodesy.noaa.gov/pub/opus-projects/ ftp://geodesy.noaa.gov/pub/opus-projects/intro.pdf ftp://geodesy.noaa.gov/pub/opus-projects/create.pdf ftp://geodesy.noaa.gov/pub/opus-projects/upload.pdf ftp://geodesy.noaa.gov/pub/opus-projects/process.pdf ftp://geodesy.noaa.gov/pub/opus-projects/manage.pdf etc. Yes indeed, I carry weights with me when using a bipod and rover pole for long static sessions. Not a problem for RTK though. I use the same weights one would use for a canopy. I bought them at the local Dick's sporting goods store. They're inexpensive (I think it was $20 for a pack of four) and the slide in opening on the weights fits the bipod legs nicely. Quick on and off. I also have a more expensive standalone fixed height SECO tripod that I picked up off Ebay. Even it sometimes must be weighted down when used on concrete or asphalt. The legs were too big for the openings on canopy weights, so I bought some leg clamps off Ebay and use my ADM Accessories (telescope) weights. I can also substitute non-skid rubber feet (from Home Depot) for the leg spikes when needed.

Hi Bill. I've also collected a few old GPS L1/L2 survey grade receivers. Are you using a regular surveyor tripod and a tribrach? It might be easier to use a two meter fixed height rover pole (with level vial) and a bipod (or tripod). I've found local HARN/HPGN stations to be fairly useless as anything other than a rough check. They're all HPGN 1991.35 and it doesn't appear possible to convert between HPGN 1991.35 and NAD83 (2011) Epoch 2010.0 with high accuracy here on the West coast. The best the NGS tools can do is 4-5 centimeters. Perhaps there's some unmodeled motion in there somewhere. In any case I can obtain more accurate positions than that by resurveying the station. In addition to OPUS, have you tried submitting your RINEX data files to: 1. NASA APPS 2. CSRS-PPP 3. Trimble CenterPoint RTX Post-Processing I've also tried SCOUT, but can't seem to get it to actually process uploaded RINEX files. Have you signed up for OPUS Projects yet? If not, NGS sometimes have upcoming OPUS Projects training online webinars available that you can sign up for and gain access at zero cost. OPUS Projects is the bomb.

The epoch is extremely important over here in California where we have significant velocity caused by plate tectonic motion and occasional large earthquakes. I find that HTDP datum and epoch adjustments of existing CSRS 1991.35 HARN/HPGN monuments don't give me the same warm and fuzzy feeling that a new current measurement does. GEOCON adjustments are even worse. I'm talking about differences of two or three tenths of a foot when comparing an HTDP datum and epoch adjusted position to a newly measured position (24 hours of raw data static post processed through OPUS Projects using either rapid or final orbits). I gather HTDP just isn't capable of that level of accuracy. I've been playing around with OPUS Projects lately. I think it's the bomb. If you haven't already done so, sign up for one of those online OPUS Projects webinars so that you can get access to OPUS Projects. If you set it up correctly it does a better job than either OPUS or OPUS-RS. I haven't seen you post anything here yet about vertical height datums such as leveled NGVD29 vs. leveled NAVD88 vs. GEOID12B or the new gravity model based one coming out in 2022 or whenever.

In the area where I live the velocity is about 39mm/year to the Northwest. Because of that, whenever specifying a location one must also specify a reference frame (datum and epoch). For example, "NAD83(2011) (Epoch 2010.0)" and "NAD83(NSRS2007) (Epoch 2011.0)" and "IGS08 (Epoch 20nn.nnnnn)" are reference frames I frequently deal with. One can use NGS's HTDP tool to convert between them. One thing I find to be unsettling is that even when using the same reference frame, different processing tools give different results. For instance I can collect 24 hours of static data and post process it using NGS OPUS-S, NASA APPS, CSRS-PPP and RTKLIB. Each one gives a different result both for the position and for the estimated accuracy of that position. And while NGS OPUS-S may appear to be optimistic (sub centimeter), NASA APPS appears to be wildly optimistic (sub millimeter).

I hate to dredge up an old thread, but what the heck... Thanks to Ebay, some eight years later (!), I now have dual frequency survey grade RTK ability as well. Being able to do RTK is the bomb. Being able to use the local RTN and not needing my own separate base is a bonus. (But I also have my own base to play with.) If you want better accuracy you can do a static survey, collect data for somewhat over four hours and submit your data online to OPUS for post processing. Repeat that at different times on several different days for comparison purposes.

Yes, I forgot that this is a benchmark hunting forum, not a positioning forum. Sincere apologies all.

I just have to laugh and shake my head when people write that the difference between NAD83 and WGS84 (G1150) is insignificant. It is only insignificant if you're using a typical consumer GPSR with a crappy antenna, not collecting raw data and are not post processing. I'm measuring station velocity caused by plate tectonic motion. In that application the difference is huge! Not just the datum, but also the epoch. BTW, my total investment to reach the level of accuracy needed to monitor plate tectonic motion has been about US$1000. That includes GPSR ($350), geodetic tripod ($250), geodetic antenna ($250), coax cables ($50) and dc bias injector ($100). (The tripod and antenna are both used.) I collect raw data 24 hours at a time and post process using RTKLIB and nearby CORS reference station observations. I've recently measured the benchmark in my backyard as moving about 34mm/year to the Northwest. Assuming that this motion has been going on continuously back in time for eons, then that adds up to just under a meter of movement since I originally placed the benchmark (when I was much younger). It was my lifelong dream to be able to measure this and how I've done it.

For a very long time I was using a Radial-Larsen GPSNMO02 antenna with my old Streetpilot III. The Radial-Larsen antenna required 5V DC bias, which the Streetpilot III was happy to provide. Awhile back I upgraded to a Streetpilot 2730 which only puts out 3V DC bias on the antenna. The Radial Larsen antenna stopped working. I'm now using the Antenex GPSU15M found at http://www.tessco.com/products/displayProd...p;subgroupId=60.

Are they even Garmin authorized retailers?

So then is there no point at which the receiver gets overloaded? If this was a radio, excess amplification on the antenna inputs would just add more noise and would likely also cause front end overload and desense. Does that never happen with GPS receivers? 55db seems like a lot. Also, that's 55db with reference to what? It doesn't say 55dbi or 55dbd, so is that 55db relative to the same reference as the 28db is? Who knows...