Elevation Question Hlo574

[Grasscatcher]

Maybe some of you Pros can answer...

 

We are "in between" seasons right now so I can't get there to take pictures. Previously I have ridden a snowmobile right up to the exact spot of this BM and kicked snow off the marker to show others the elevation.

 

HLO574 is a BM on top of Baldy Cinco, which according to all available maps, signs,etc, is 13383 ft elevation.

On the GC datasheet it says the elevation is 13390 ft. On the original NGS datasheet it mentions 13390, 13420 ft Elevation, and also in the superseded survey control it mentions 13383 ft.

 

GC says the name is CINCO RESET

 

On the disk itself, 13420 ft Elevation is stamped.

 

On the original NGS Datasheet it says the markings are CINCO 1904-1956.

 

AM I CONFUSED ....or WHAT? Is there any way to decipher the history?

 

Maybe on a long hike this Summer.......

Edited April 16, 2006 by Grasscatcher

[+Black Dog Trackers]

Grasscatcher -

 

From my limited knowledge, here is the timeline for HL0574:

 

1. There was a survey station here in 1904, probably the mountain top itself.

2. In 1956 the USGS (not the same agency as the NGS or even its predecessor USG&GS) set a benchmark disk for vertical control. (The topo maps are made by the USGS and they set benchmarks while making the maps.)

3. In 1958 a description of the mark was written by the USGS.

4. In 1988 the old NGVD 29 geoid system (datum?) was updated by NAVD 88. This resulted in changes to elevations because of re-calculation and the height became 13390 instead 13420.

5. Sometime after 1958, some survey party used the monumentation and 1956 disk for horizontal control. This may be the same time that the mark entered the USC&GS (now NGS) database as a horizontal control.

 

The reason I say the disk is now horizontal control can be seen just below "Altitude: 13390" in the datasheet where it says "location is ADJUSTED". The reason I say the disk was orignally vertical control is that the 1958 description calls the station a "BENCH MARK DISK". Technically, a bench mark is strictly for vertical control, although we use the slang "benchmark" here to indicate either horizontal or vertical control.

 

The mark isn't listed by the NGS as having an accurate vertical elevation; they took the USGS's word for it.

This is not to say that the elevation is not accurate - only that the NGS does not say how accurate it is.

The mark is used in the NGS database to provide very accurate horizontal coordinates only.

[+jwahl]

I am not very expert on leveling, but will take a first shot at this.

 

1) The superceded value shown was 13383. Note that was determined by vertical angles, not levelling and is shown to the nearest foot. That would also have been based on an old datum (NGVD29).

 

2) I believe that when the original work was done a preliminary calculation is used to 'mark' an elevation on the cap. That would be where the 13420 would have come from. It would be based observations done before the final network was computed, and so is some degree of an approximation. Again all on an earlier datum. (NGVD29)

 

3) The current elevation given is a conversion from the 13383 value and as is stated is the result of applying the vertcon datum conversion (not precise) to the NGVD29 value in 1) to come up with the NAVD 88 value 13390, whenever the datasheet was updated after that datum was instituted and computed for this area.

 

- jlw

 

Maybe some of you Pros can answer...

 

We are "in between" seasons right now so I can't get there to take pictures. Previously I have ridden a snowmobile right up to the exact spot of this BM and kicked snow off the marker to show others the elevation.

 

HLO574 is a BM on top of Baldy Cinco, which according to all available maps, signs,etc, is 13383 ft elevation.

On the GC datasheet it says the elevation is 13390 ft. On the original NGS datasheet it mentions 13390, 13420 ft Elevation, and also in the superseded survey control it mentions 13383 ft.

 

GC says the name is CINCO RESET

 

On the disk itself, 13420 ft Elevation is stamped.

 

On the original NGS Datasheet it says the markings are CINCO 1904-1956.

 

AM I CONFUSED ....or WHAT? Is there any way to decipher the history?

 

Maybe on a long hike this Summer.......

Edited April 17, 2006 by jwahl

[Grasscatcher]

Thanks to both responders.

 

May still have to take a hike up there this Summer and get some pictures. The air is rare and the views are spectacular. I'll also see if I can "get" the ref markers too.

 

Thanks again

[+Black Dog Trackers]

Grasscatcher -

 

When you go back up there, be SURE to bring the information from the NGS datasheet's box score to help you find the ref marks.