Is GPS vertical accuracy the same or similar to horizontal accuracy? Was checking some places today trying to correlate river stage to elevations. It appears the GPS elevations are 30+ feet higher than expected elevations based on USGS topo maps and river stages.

Depending on your GPS it could be very inaccurate.
Altitude is an atmospheric function. Your GPS is just importing map data based on approximate positioning.

It's a fairly new Garmin 64ST.

Depends on the topo source but I would believe that before the GPS unit. Now if you are in an area with lidar mapping the maps produced (ersi gps) are very accurate. In the past I have had gps units off as much as 30 yards, so you really have to watch it. Not a big deal when just messing around but a very big deal when using for research, engineering, or timber harvest.

My work is ecology of floodplain forests and outside of newer lidar maps I wouldn't trust the accuracy of a handheld unit....but just depends the level of accuracy you're looking for.

Semo , is there a consumer grade gps you would recommend?

Wow, did a little google search and after reading up on geoid and ellipsoids, my head hurts.

Honestly, I'm not an expert on them. For me it is more about ease of use and mapping software when I use one for non-work things. You would be better 9ff checking online reviews. I just don't have experience with all the models.

When marking elevations where I need to be more precise I still use surveying transits and a stick. But in areas within refuges and such the advancements in elevation mapping is pretty awesome.

You gotta get the GPS SAASM module.

we use mapping grade (3 meter, but sub-meter has become the standard) and have never been accurate on elevation, like 3x worse or more than horizontal. I’m not sure there is a consumer grade gps that is, really. High accuracy gps is also high dollar gps...$75k plus. It can be done, but not cheap, without a good source of parts and knowledge.

In open skis, vertical is typically 1.7 to 2 times your horizontal accuracy.

A lot of cell phones are fairly accurate. I think it is being used in large cities to help locate (get a good starting point) what floor folks may be on. This cuts down on emergency response times.

With today’s ever changing technology Cell phones can also use things like Bluetooth beacons, Wifi, and a plethora of other things to determine your Z value in buildings. Savings seconds saves lives in an emergency.

Unless you are using some very high end survey grade units I wouldn’t pay much attention to the elevation returned by a handheld unit. It can be close, but close may be 30’. I have better luck using a good topo map service and reading the contours on it. If you are fortunate and have good lidar data that is your best bet.

Better luck using judgement

This is coming from a guy who works for the USGS and oversees the gaging network...

USGS steam gages have historically used “gage datum” which is not the same datum, such a NAVD 88, you see on topos. Back in the day when a new stream gage was being built, a tape down to the thalweg (deepest portion) from the bridge pier would be used to roughly estimate datum. You would do this because you never wanted your gage height (water level) to go below zero. So say you tape down 50’ to the deepest portion of the body of water. You would then set your reference mark on the bridge 60’ to give you some wiggle room. Then, whatever the water surface read, was the gage height. All this is hypothetical by the way. There are and were several different methods to establishing “gage datum”. The good news is, all “gage datum” can be easily converted to NAVD 88 through modern surveying techniques.

The USGS as a whole is aware of the issue you are talking about because it is an issue all over the US. In 2022, the National Geodetic Survey will release a new datum, changing everything. By 2022, the current goal is for all USGS stream gages to be converted to NAVD 88, which can in turn be translated to the new 2022 datum. This means by that time, all water surface elevations displayed on the web will be accurate when compared to a map.

Honestly, it’s a long discussion that I’d be willing to have with you, but it involves a lot of intricacies that is hard to relay on a message board. I hope this helps with your question some.