I have a distinct recollection of reading somewhere about differences between PACS and SACS monuments and other NGS monument reference systems. I can't find any info relating to this and am beginning to wonder if I am imagining things. I think it may have related to vertical datum issues, but can't say for sure. Can anyone help me?
My experience with PACS (Primary airport control) and SACS (Secondary airport control) was that they wanted it referenced to WGS84, but that was several years ago.
Here's what NGS has on their web site:
Airports with NGS established PACS and SACSPACS and SACS are a control station established in the vicinity of an airport and tied directly to the National Spatial Reference System. This control consists of permanent marks with precisely determined latitudes, longitudes and elevations. PACS and SACS are designated by the National Geodetic Survey and must meet the specific siting, construction, and accuracy requirements.
I pulled up the PAC and SACS for one of our regional airports, and everything was in the current epoch of NAD83. They didn't even show elevations for any of them because they're in a subsidence area.
Here's what the FAA says on the subject: https://airports-gis.faa.gov/public/OnlineHelp/contents/pacsandsacs.html
Jim in AZ, post: 360928, member: 249 wrote: I have a distinct recollection of reading somewhere about differences between PACS and SACS monuments and other NGS monument reference systems. I can't find any info relating to this and am beginning to wonder if I am imagining things. I think it may have related to vertical datum issues, but can't say for sure. Can anyone help me?
See http://www.ngs.noaa.gov/AERO/Genspecs_A/Volume%20A_Attachment%2011-19.pdf
Note longer observation times, etc
Finally found the note I was remembering - its on the PACS datasheets:
"GPS derived orthometric heights for airport stations designated as PACS or SACS are published to 2 decimal places. This maintains centimeter relative accuracy between the PACS and SACS. It does not indicate centimeter accuracy relative to other marks which are part of the NAVD 88 network."
I'm not sure I fully understand this...
Does this mean the PACS and SACS are already on a GPS plus geoid model for a vertical datum? So they don't relate directly to the old NAVD88 leveled network. That's where we're headed with GRAV-D, but I didn't know that the airports were already doing that.
Bill93 is correct. As the observation scheme and adjustment guidelines in the document in my previous post do NOT require ties to NAVD88 benchmarks how else can an orthometric height be derived?
Note that the document's adjustment section does include an option to include NAVD88 benchmarks. In some cases, some of the airport monuments might already be benchmarks. It was common practice to run levels to airports in the vicinity of level projects. Also there was a USC&GS program to perform Airport Obstruction Chart surveys throughout the US. These surveys required the establishment of horizontal and vertical control monuments tied to the national system. The program dates from the late 1940's. Airports in the program were a rather small subset of all airports in the US.
I understand the PACS/SACS program was to tie all size airports to the national network. It as not possible with the federal resources.
As Bill93 notes, this is the way orthometric heights will be determined for government work in the future. The discrepancy between NAVD88 heights on monuments and those derived via high-accuracy ellipsoid heights and an improved geoid model is an issue. Unlike some I have difficulty being confident that ties to monuments not verified in decades will yield accurate results. The practice of determining heights with respect to a single benchmark is even more troubling.
The deficiencies of NAVD88 are now well understood. The cross-continent tilt should enough to recognize the need to the new approach.