Has anyone here familiar with the Cambridge City Base datum as shown in the image below?
My measurements using CORS are showing that NAVD88 is lower than the Cambridge Base by 11.50', not higher as indicated in the notes of a surveyor who appears to be out in business any longer.
MassDOT has 11.66 in their datum conversion from '88 to Cambridge Base.?ÿ Cambridge is definitely lower than NAVD88 which is how I would read that image you posted.?ÿ I have run into some differences in the GPS CORS stations around the eastern MA area that can pop up compared to some old first order benches so the 11.50 to 11.66 is not outside those deltas.?ÿ?ÿ
Has anyone here familiar with the Cambridge City Base datum as shown in the image below?
My measurements using CORS are showing that NAVD88 is lower than the Cambridge Base by 11.50', not higher as indicated in the notes of a surveyor who appears to be out in business any longer.
Are you saying that the NAVD88 heights are numerically smaller than the Cambridge Heights? In that case the sketch is correct. Or are you saying that the zero of NAVD88 is below the zero of Cambridge?
Those diagrams are always a bit confusing at first.?ÿ
Clearly it's showing the NAVD88 0 elevation to be above the Cambridge 0 elevation which will make Cambridge bench marks "higher" than the NAVD88 bench marks, even though the NAVD88 base is higher than the Cambridge base. Hopefully, that clears it up, also the little hand is helpful.?ÿ
I was told by a local surveyor the intent was to not have negative elevations. So they created Cambridge datum at higher elevation. This is what my measure shows. But the sketch shows opposite.?ÿ
@leegreen I think the sketch shows the intent you described not the opposite. An NAVD'88 elevation of minus 7' would fall between the top line of the sketch and the bottom line of the sketch. So in order to avoid negative elevations they made the Cambridge 0' elevation lower than NAVD'88. But if you were to measure 1 point the Cambridge elevation would have a higher numerical value than the NAVD'88 elevation. The sketch appears to be correct.
The Cambridge datum would?ÿ have to be higher the 88 datum to eliminate negative values.
When I setup with CORS on 88 datum I see values lower than Cambridge. This confirms the sketch from contract drawings is wrong. Also in harmony with the intent.
Lee the sketch is correct if you use a 0 elevation for the line showing vertical datum representing NAVD88 and 0 elevation for the line showing vertical datum representing Cambridge datum. Then measure a bench mark on NAVD88 of 50.00' it would be 61.50' on the Cambridge datum and I believe that's what you are seeing.?ÿ
Lee the sketch is correct if you use a 0 elevation for the line showing vertical datum representing NAVD88 and 0 elevation for the line showing vertical datum representing Cambridge datum. Then measure a bench mark on NAVD88 of 50.00' it would be 61.50' on the Cambridge datum and I believe that's what you are seeing.?ÿ
This is the correct interpretation of the sketch. There is a similar sketch in the MassDOT (formerly MassHighway) survey manual showing a great number of local datums relative to NAVD88. When the base (0 elevation) of the datum is below the base of NAVD88, you have to add the offset to the NAVD88 elevation value in order to be in the local datum. Subtract when the datum base is above NAVD88.
@leegreen Ok here is a different way to visualize the concept. If your 2 datums were "Mean High Water" datum and "Mean Low Water" datum, would you not sketch the high water datum line above the low water datum line and wouldn't you say the high water datum is above the low water datum? Yet they still produce the same numerical relationships as NAVD'88 vs Cambridge.?ÿ
Here's a footnote: "1Cambridge City Base datum is a standard vertical datum used by the City of Cambridge. This datum is 11.65 ft above the national standard vertical datum NAVD88, and 11.95 ft above the mean sea level in the Boston area."?ÿ
Here's a publication from Boston that says that "NAVD88 can be converted to BCB by using a conversion factor of NAVD88+6.46 feet." Now as every student I ever taught knows, "factors" are multipliers and added numbers are "terms." Maybe say, "... using the formula BCB = NAVD88 + 6.46 feet."
I'm with Lee. I would interpret that diagram as saying that NAVD88 is above the Cambridge datum by 11.66 feet everywhere. But I think it's saying that there's an 11.66 foot difference between the two with the two arrowheads signifying tat whoever drew the diagram didn't know which way was up.
