Here is the first image showing the clear weather in 2015
when the GPS survey was performed on Denali. The one
item in this photo is a survey monument on a rod stuck in
the snow by Jeff Yates in 1989. It’s still there today but could
easily be moved around if anyone wanted to move it slightly.
It’s at the highest elevation on Denali.
Image courtesy of CompassData.
NATIONAL HEIGHT SYSTEM
Gravity vs. Geometry
N
ational height systems have
long been based on the
rules of gravity to determine
orthometric heights (H) and model the
directions in which water will flow across
the earth’s surface. Until the late 1990s,
the National Geodetic Survey (NGS) had
relied on conventional line-of-sight survey
measurements to provide a consistent,
accurate national network for latitude,
longitude, height, velocity, and shoreline
delineation. To establish a height system
across the nation, crews of geodetic
surveyors traditionally trekked many
miles along a designated route, carrying
surveying equipment, carefully leveling
their instruments so that horizontal linesof-sight would be level, i.e., perpendicular
to the local direction of gravity, and taking
geodetic leveling measurements every
hundred yards or so. For the network’s
horizontal component, an NGS crew
would install a permanent brass disk
marking a geodetic reference point and
construct a temporary steel tower over
the point. Then an NGS geodesist and
assistant climbed the tower, leveled their
survey instrument, observed other distant
reference points by line-of-site measurements, computed coordinates, and
eventually built a system of more than a
million reference points across the nation.
With the advent of the Global
Positioning System (GPS), NGS knew
there was a better way. Geodesists had
long known the importance of geoid
models and the differences between
orthometric heights and ellipsoid
heights, but many land surveyors
BY DR. DAVID F. MAUNE
Displayed with permission • LiDAR Magazine • Vol. 6 No. 7 • Copyright 2016 Spatial Media • www.lidarmag.com
did not. With GPS, surveyors started
consulting the geodetic formula and
some version of the graphic at Figure 1
to understand that conventional land
surveys follow the rules of gravity and
determine orthometric heights (H)
above the undulating geoid (equipotential surface) whereas GPS follows
the rules of geometry, independent of
gravity, and determine ellipsoid heights
(h) above the mathematically-derived
reference ellipsoid.
NGS first completed the horizontal
component of the National Spatial
Reference System (NSRS) and then
turned its attention to the vertical
component. In November of 1997,
NGS published NOAA Technical
Memorandum NOS NGS-58,
Guidelines for Establishing GPS-Derived
Ellipsoid Heights (Standards: 2 cm and
5 cm) but was uncertain as to the cost
benefits of GPS when compared with
Conventional land surveys
follow the rules of gravity to
determine orthometric heights
(H) above the undulating geoid,
whereas GPS surveys (including
land and airborne GPS) follow
rules of geometry to determine
ellipsoid heights (h) above the
mathematical ellipsoid.
traditional surveying technologies.
In 2008, NGS published NOAA
Technical Memorandum NOS NGS-59,
Guidelines for Establishing GPS-Derived
Orthometric Heights.
In 1997, NGS contracted with
Dewberry to execute the National
Height Modernization Study, with the
following goals:
⦁⦁ Identify and document user requirements for height data, including
those requirements utilizing both
vertical and horizontal data
⦁⦁ Identify and document major users
and application of the National
Height System (NHS) and GPSderived height data
⦁⦁ Identify and recommend the best,
most cost-effective actions that
meet the documented user requirements, taking into consideration all
technologies available
⦁⦁ Evaluate the estimated costs to implement the recommended actions, and
their benefits to the nation
“ ewberry
D
subcontracted with
12 survey firms to
obtain case study
reports including
lessons learned
from GPS elevation
surveys compared
with traditional
leveling. ”
Figure 1: The relationship between ellipsoid heights (h, purple), orthometric heights
(H, green) and geoid undulations (N, orange)
To execute these tasks, Dewberry
participated in NGS-sponsored User
Forums to obtain preliminary user
requirements for elevation data. We
interviewed representatives of 20 major
categories of users of elevation data
to determine their requirements for
DEMs, static and real-time kinematic
elevation surveys. Dewberry subcontracted with 12 survey firms to obtain
Displayed with permission • LiDAR Magazine • Vol. 6 No. 7 • Copyright 2016 Spatial Media • www.lidarmag.com
Figure 2: Front cover of the National Height
Modernization Study report prepared by
Dewberry.
case study reports including lessons
learned from GPS elevation surveys
compared with traditional leveling.
We subcontracted with other survey
firms to obtain time and cost-benefit
comparisons between GPS elevation
surveys (to NGS 2-cm standards) and
traditional leveling (second order, class
1) under controlled conditions. We
evaluated differential GPS, LiDAR and
IFSAR technologies for mass production of elevation data; and we reported
major findings and recommendations
for development of a modernized
National Height System in the U.S. The
first three major findings in our 1998
report (Figure 2) were as follows:
1. America needs a reliable and
efficient means to determine
absolute elevations, relative
only to the earth’s center. (In
1998, GPS was unreliable for
civil users because of Selective
Availability (SA) then in effect
which deliberately degraded GPS
signals to civil users; and existing
benchmarks did not have absolute
accuracy, but relative accuracy.)
[Solution: Differential GPS (DGPS),
Continuously Operating Reference
Stations (CORS), improved geoid
model, and elimination of SA.]
2. Based on DGPS and CORS,
America needs nationwide
implementation of a standardized
vertical reference datum as the legal
basis for elevation data. [Solution:
Height Modernization Program
funding for implementation of
the modernized National Height
System based on NAVD 88.]
3. America needs high accuracy, high
resolution DEMs based on NAVD
88. [Solution: Funding for LiDAR
and/or IFSAR DEMs nationwide.]
won the NOAA/NGS contract for
Remote Sensing, Mapping and Charting
Services which has included the acquisition of gravity data for NGS’ Gravity
for the Redefinition of the American
Vertical Datum (GRAV-D). GRAV-D’s
goal is a refined gravimetric geoid model
that enables GPS-derived elevations
accurate to 2 cm in the National Spatial
Reference System (NSRS) update of
2022. Between now and then, most of us
practitioners will need to learn how this
impacts our calculations of elevations
(orthometric heights) from LiDAR
sensors which collect ellipsoid heights—
not orthometric heights.
When I studied geodesy at The
Ohio State University over 40 years
“ hen I studied geodesy at The Ohio State
W
University over 40 years ago, we studied the use
of spherical harmonics for geoid modeling.
Dewberry has subsequently worked
closely with NOAA and other agencies
to act upon these findings. Dewberry
subsequently executed several of NGS’
Height Modernization contracts, to
include one task order where we operated 25 geodetic grade GPS receivers
simultaneously for 48 hours so we could
compare different combinations of
observation times and baseline lengths
to achieve different accuracies.
In 2005, Dewberry won our first
Coastal Geospatial Services Contract
(CGSC) with what is now NOAA’s
Office for Coastal Management (OCM)
and we have recently negotiated terms
for the CGSC-3 contract. In 2012, we
”
ago, we studied the use of spherical
harmonics for geoid modeling. Only
the very smartest in our class (not me)
understood spherical harmonics, one
of the main reasons why I chose to
specialize in photogrammetry instead.
In 2022, all of us producing LiDAR
datasets, whether we realize it or not,
will be placing our trust in a handful
of very smart geodesists at NGS
who understand the complexities or
gravity as well as the use of spherical
harmonics and other mathematical
tools for the new gravimetric geoid.
They are the “brain surgeons” of the
geospatial community, and I look
forward to their results.
Displayed with permission • LiDAR Magazine • Vol. 6 No. 7 • Copyright 2016 Spatial Media • www.lidarmag.com
Today, if you ask Google: “What is
the National Height System in the U.S.?”
you will be directed to the NGS page on
vertical datums which states: “A vertical
datum is a surface of zero elevation
to which heights of various points are
referred in order that those heights be
in a consistent system. More broadly,
a vertical datum is the entire system of
the zero elevation surface and methods
of determining heights relative to that
surface.” In just six years from now, all of
us in the LiDAR industry will be adjusting our processes so as to correctly use
the refined gravimetric geoid model. Stay
tuned. You will be hearing a lot more on
this subject in the years ahead.
Did you know?
For snow-capped mountains, it has
been common practice to map the
elevation of the top of the snow on
mountain peaks. In 2015, Dewberry
and CompassData, Inc. surveyed
the new elevation of Denali (formerly
Mt. McKinley) at 20,310 feet, paid
for by USGS and NGS. In 2016,
USGS tasked us to return to Alaska
with ground penetrating radar; we
determined that the rock at Denali’s
peak is about 4.15 m below the snow,
but its official elevation (at the top of
the snow) remains unchanged. How
cool is that?
Dr. David Maune is an Associate Vice
President at Dewberry Consultants LLC
where he is an elevation specialist and
manages photogrammetric, LiDAR, IFSAR
and sonar projects for USGS, NOAA, FEMA,
USACE, and other federal, state and county
governments. He authored the National
Height Modernization Study referenced
in this article. He is a member of NOAA’s
Hydrographic Services Review Panel
(HSRP). He specializes in independent QA/
QC of LiDAR data produced by others and
is perhaps best known as the editor and
primary author of the 1st and 2nd editions
of Digital Elevation Model Technologies
and Applications: The DEM Users Manual
published by ASPRS. He is a retired Army
Colonel, last serving as Commander and
Director of the U.S. Army Topographic
Engineering Center (TEC), now the Army
Geospatial Center (AGC).
From June 2016 when CompassData used ground-penetrating radar to determine the depth of the snow and ice on Denali.
There was a blizzard, with high winds and very low temperatures and white-out visibility conditions; yet they got the job done.
Image courtesy of CompassData.
Displayed with permission • LiDAR Magazine • Vol. 6 No. 7 • Copyright 2016 Spatial Media • www.lidarmag.com