I need some ideas of how to explain to some people requiring my services the need to specify the vertical datum they want their elevations in. The problem is, they're not surveyors, they don't understand a thing about why it would make any difference. To them, it's all 'Mean Sea Level'. 'We want you to deliver MSL elevations to within 1' on 16 locations. Of course there isn't a reliable benchmark within 50 miles of half of these places. I've been through this before and it's so frustrating for both sides. They have no clue what they're asking for. Any ideas? I don't want to make anyone feel stupid, I just want a simple way to explain to them why it would make a difference. The difference between NGVD29 (MSL) and NAVD88 orthometric heights around here is about 6'. To these people a geoid is something you treat with Preparation-H.
What to do .... :bored:
Being in an area that is rather void of actual verified BMs, the method I learned to arrive at MSL values were to locate as many spot elevations we could gather as found on USGS maps.
Since they were to the nearest foot, an average would be computed and used as the correction to MSL.
Many turning points and grades are included in the original level runs following RRs and other open roadways with most being destroyed by expansion and modernization.
Glad that 99.9% of my work does not require elevations.
Tell
Them
The
Facts.
If they don't understand, so be it. If they do, you have broadened their horizons about the requirements for the work involved. I would not underestimate anyone's intelligence.
Williwaw, post: 364232, member: 7066 wrote: They have no clue what they're asking for.
very true, but actually "they" are not asking nor do they care. They are buying a product to comply with some agency requirements... maybe it is better to use the short version: "to get my elevation certificate approved I need to... ".
Nobody "wants a survey", they are required to get one by somebody else. (with rare exception)
Robert Hill, post: 364249, member: 378 wrote: Tell
Them
The
Facts.If they don't understand, so be it. If they do, you have broadened their horizons about the requirements for the work involved. I would not underestimate anyone's intelligence.
I wanted to edit/addend above but got shut out.
98% of the public does not know the definition of a geoid ( unfortunately, you may find a small % of surveyors who canÛªt define the term correctly). But people do understand the advances in science and technology from many facets of their daily life. I now that I harped on this before in posts of time past, but donÛªt insult yourself (and others) by miscalculating the intelligence of others.
In Louisiana it would be a no-brainer since NAVD88 is required by statute
Couldn't be further from the sea than me, yet I still hear MSL, I joke which ocean? Which tidal station do you want me to bench from? I then explain we use a model to average sea level across the continent and that's what we derive elevations from.
Williwaw, post: 364232, member: 7066 wrote: I need some ideas of how to explain to some people requiring my services the need to specify the vertical datum they want their elevations in. The problem is, they're not surveyors, they don't understand a thing about why it would make any difference. To them, it's all 'Mean Sea Level'. 'We want you to deliver MSL elevations to within 1' on 16 locations. Of course there isn't a reliable benchmark within 50 miles of half of these places. I've been through this before and it's so frustrating for both sides. They have no clue what they're asking for. Any ideas? I don't want to make anyone feel stupid, I just want a simple way to explain to them why it would make a difference. The difference between NGVD29 (MSL) and NAVD88 orthometric heights around here is about 6'. To these people a geoid is something you treat with Preparation-H.
What to do .... :bored:
I've been holding whichever elevation is expressed on the FEMA maps for any kinda elevations connected to flood plains, recently it changed to NAVD88 locally so that makes it easier to justify as a basis, just say I'm using the system from the gov. maps.
Since you are 50 miles from a bench mark, I'm guessing that you don't have flood plain maps with any real data (bench mark system). That does make it harder, I'd probably give them some datasheets and say I'm using what is on these official government datasheet and using whichever EPOCH and GEOID that is relevant. You could even highlight the correct lines connecting your data to the datasheet.
I do all my work in NAVD unless specified differently. If they need a topo or elevations I don't even ask what datum. It's NAVD unless they say otherwise.
Williwaw, post: 364232, member: 7066 wrote: I need some ideas...
What to do .... :bored:
I would use whatever is currently utilized by your district/region/county. And make certain to identify the datum and benchmark utilized on the face of the plat.
Worst case scenario, is you modify the map to reference the alternate datum, which doesn't sound to big of a deal, as you already know the difference.
summerprophet, post: 364261, member: 8874 wrote: I would use whatever is currently utilized by your district/region/county. And make certain to identify the datum and benchmark utilized on the face of the plat.
Worst case scenario, is you modify the map to reference the alternate datum, which doesn't sound to big of a deal, as you already know the difference.
Sorry, yes this is what I meant when I said I use NAVD without asking... that is bc NAVD is what is utilized by my region.
Just saw a set of plans last week that said "horizontal datum = state plane, vertical datum = NAD83" I can't believe these morons have a license. One of the more difficult states too.
Also, you had mentioned no reliable benchmarks within 50 miles. Really? None? Nothing published by usgs or the state or DOT or cities/counties?
Not sure how you qualify as reliable, but is it has published values, and I don't have knowledge it is specifically bad, I am using it. Ignorance really is bliss.
In the event there really is nothing, I would resort to an OPUS observation, and publish it, to assist those in the future.
I wonder if you can help me understand the situation?
Living in the small island of Great Britain, our national mapping service provides 90 CORS so that you're never more than 75km (47 miles) from one. We're advised by the RICS (a professional institution) to observe for 1 hour (or 2, 4, 6) when the CORS is within 20km (or 30, 50, 100). As we have an algorithm to adjust to horizontal or vertical coordinates, we can achieve a value to a datum based upon a version of MSL to within +-5cm.
Does the NGS provide something similar? Is it country-wide?
Thank you
I do not mind when the server makes a suggestion as to what to order if I am unfamiliar with what is on the menu.
Victor.Plymouth, post: 364276, member: 11292 wrote: Does the NGS provide something similar? Is it country-wide?
Thank you
Yes they do. There is a network of Continuously Operating Reference Stations (CORS) on roughly the same spacing as you describe for Britain (some states are a little more sparse, some less).
The problem we have is that there are at least 2 national elevation datums. One established in 1929, and one in 1988.
The 1929 datum, although officially obsolete, is still used by a lot of agencies with long term historic data and/or institutional inertia. That and a lot of local agencies have there own datum.
summerprophet, post: 364261, member: 8874 wrote: I would use whatever is currently utilized by your district/region/county. And make certain to identify the datum and benchmark utilized on the face of the plat.
Worst case scenario, is you modify the map to reference the alternate datum, which doesn't sound to big of a deal, as you already know the difference.
50 miles doesn't suprise me, considering where he is, don't think I ever get quite that far from a level run, but I'm sure in areas where it's a long hike to a bench mark....
Why does the Client need the Elevation? Is it for communication? Is it for a site plan in the middle of nowhere where needing to be able to determine elevations in case your Bench Marks are destroyed? In other words, the answer is in what the Client is doing with the Elevations. Many times a Client will give a Scope and my first question is what will you be doing with this information? In order to have an accurate scope, you need to know why your survey is needed. Sometimes the datum is of less important than the invert of the closest manhole.
Near the shore? Here the difference is about a foot. If you stood at the shore at MSL NGVD and then the water changes to MSL NAVD then you are standing in a foot of water.
Sea level rise, been happening since the end of the last ice age.
Planet earth, an ooey gooey mud ball that we often mistake to be stable.
Sent from my SM-G900V using Tapatalk
Mark Mayer, post: 364279, member: 424 wrote: Yes they do. There is a network of Continuously Operating Reference Stations (CORS) on roughly the same spacing as you describe for Britain (some states are a little more sparse, some less).
The problem we have is that there are at least 2 national elevation datums. One established in 1929, and one in 1988.
The 1929 datum, although officially obsolete, is still used by a lot of agencies with long term historic data and/or institutional inertia. That and a lot of local agencies have there own datum.
Bingo.
The problem comes in when the folks I'm doing the work for don't know the difference (or care) and I suspect are reluctant to reveal their ignorance on the subject by seeking out the answers I need in order to provide them with the specific answers within the tolerances they're specifying. They throw around 'MSL' like every elevation they've ever seen is related to this and they don't really have a clue. Several years ago I spent a couple of weeks scouring the region seeking out the old benchmarks that were shown on the USGS quad maps so I could provide them with the specified NGVD29 elevations on remote communications sites as they requested. Most of the old benchmarks had disappeared over the years, but I did manage to find enough of them that I could begin to relate the old NGVD29 stuff to NAVD88, but it was a real stretch. So long story short, I need to quickly and effectively communicate to the higher up that their is a difference and they need to be sure which of the two they need if they expect me to certify with my professional license that they are correct and in the process avoid making anyone feel stupid. Since for the most part all of this work will be done using static GPS, I'm very tempted to just give them a NAVD88 ortho height, specify the epoc and geoid and leave it at that, since it's all the same to them ...
spledeus, post: 364300, member: 3579 wrote: Near the shore? Here the difference is about a foot. If you stood at the shore at MSL NGVD and then the water changes to MSL NAVD then you are standing in a foot of water.
Sea level rise, been happening since the end of the last ice age.
Planet earth, an ooey gooey mud ball that we often mistake to be stable.About 5.75' here from what I can make out, pushing 6' farther inland.
Why does the Client need the Elevation? Is it for communication? Is it for a site plan in the middle of nowhere where needing to be able to determine elevations in case your Bench Marks are destroyed? In other words, the answer is in what the Client is doing with the Elevations. Many times a Client will give a Scope and my first question is what will you be doing with this information? In order to have an accurate scope, you need to know why your survey is needed. Sometimes the datum is of less important than the invert of the closest manhole.
They seem very reluctant to tell me anything, but yes, communications related. Some of the sites are mountain top and require a helicopter to access.
I don't know why this aggravates me so much!
Ahhh ... ignorance is bliss.
