The Geoid Slope
Validation Survey of 2014

GSVS14

The Geoid Slope Validation Survey of 2014 (GSVS14) represents the second survey conducted by the National Geodetic Survey (NGS) to test the accuracy of the gravimetric geoid model, which will be the basis for the next American Vertical Datum. GSVS14 was conducted in Iowa, predominantly along U.S. Highway 30, from Denison to Cedar Rapids. The official survey line was approximately 200 miles (325 km) long, containing 204 survey benchmarks. This location was selected because it was a medium-high, relatively flat, and gravimetrically complex area ranging from 740 feet to 1,440 feet above sea level.

As of June 2015, all project surveys described below are complete. All GSVS14 survey data collected will be analyzed and released by NGS in 2015. A subsequent report will be published when results of the survey are finalized.

Image of GSVS14 survey line. Map data ©2012 Google

Project Surveys

Below is a list of surveys completed by NGS:

Long Session GPS at Official Points
First order, class II geodetic leveling connecting the entire set of Official Points
Gravity to include:
- FG-5 absolute gravity at Gravity End Points
- A-10 absolute gravity at Gravity End Points and all Gravity Anchor Points
- Lacoste-Romberg relative gravity connecting Gravity End Points and also all Official Points
- Lacoste-Romberg based gradients of gravity at Gravity End Points and Gravity Anchor Points
Astro-geodetic deflections of the vertical using the CODIAC camera (CODIAC Eccentric Camera Points)
Airborne gravity over entire line
Real Time Kinematic (RTK) GNSS using Iowa Real Time Network: Performed as 1 minute long RTK observations at CODIAC Eccentric Camera Points.

Data Analyzed

Leveling
GPS
Deflections of the Vertical (DoV)
Gravity (aerial, terrestrial, relative, gradients)
Light Detection and Ranging (Lidar)
Airborne gravity

Survey Line Specifications

Click the image for a larger version.

Minimal Trees (GPS)
No major bridges/water crossings (Leveling)
Along roads (ease of access)
Reasonable travel/per diem expenses
Mostly cloud-free nights (astrometric observations)
Under GRAV-D coverage (or will be)
Marks can be set within 40' of the shoulder of a road
With at least some existent bench marks/NAVD 88 level lines
A "significant enough" geoid slope to be interesting
"High and flat" or possibly "medium high, more-or-less flat and gravimetrically complex" topography
Begin and End at more-or-less populated areas, for a variety of P.R., travel, housing and other issues
Availability of hotels at locations *along* the line, not just at the endpoints

Definitions

In order to describe the survey, the following groups of points must first be defined:

Official Points: These are exterior passive geodetic control marks, spaced approximately every mile along the survey route, capable of being occupied by a GPS fixed height pole tripod, a level rod and a Lacoste-Romberg relative gravimeter.
Gravity Anchor Points: These are a subset of the "Official Points," located at approximately every 7th point. These points were occupied by the same instruments as the "Official Points" but were also occupied by a g-pod (for gravity gradients) and an A-10 absolute gravimeter.
Gravity End Points: These are separate from "Official Points," consisting of three building-interior points, set within 1 km of the first, middle, and last "Official Points" of the survey. They were occupied by a g-Pod, an FG-5 absolute gravimeter, a Lacoste Romberg relative gravimeter and a level rod.
Leveling Points of Opportunity: These are separate from "Official Points," and are exterior passive geodetic control marks which were not chosen to become "Official Points," but which are within 500 meters of the survey line and which have pre-existing NAVD 88 published heights. These points were surveyed for "blue booking" purposes.
CODIAC Eccentric Camera Points: These are exterior passive geodetic control marks of a temporary nature (PK nail, spray paint or other) which lie within 100 meters of a corresponding "Official Point." They will be set by the CODIAC camera crew on-the-fly when the CODIAC camera system cannot be driven directly over an Official Point.