PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
March 2014
253
Mapping India on Large Scales -
A Quick and Viable Solution
Krishna Kumar Naithani
Abstract
For most of the mapping and engineering survey projects, the
heights are still required above Mean Sea Level (
MSL
), whereas
the heights given by Global Positioning System (
GPS
) are in
terms of the
WGS84
ellipsoid. The heights above
MSL
are called
Orthometric Heights. Presently for India and many other
countries, who have not yet established their own geoid, there
is no method to convert the ellipsoidal heights to Orthometric
heights. For such countries, the only way available to obtain
Orthometric heights is through establishing a dense network
of leveling lines that is very time consuming, cumbersome,
and manpower- and cost-intensive. An alternative solution
has been proposed that requires only a very few ground
control points to be provided by
GPS
and a skeletal leveling
network for large scale photogrammetric mapping without
compromising accuracy. The concept has been validated
with the aid of two test areas. It is shown that by adopting
the proposed method, there is a significant reduction in the
required number of
GPS
control points by more than 55 per-
cent and the number of
MSL
height control points (as also the
preprocessing/postpointing effort) by as much as 90 percent.
Introduction
India was one among the very few countries (including the
advanced ones) to have completed topographic mapping on
1:50 000 and 1:25 000 scales, employing photogrammetric
techniques, about four decades ago. Since then, there has
been tremendous progress in the field of imaging and geo-
matics by way of the Global Positioning System (
GPS
), Inertial
Measuring Units (
IMUs
), Large-Format High-Resolution Aerial
Digital cameras, etc. Due to these developments, several
countries have standardized the scales as large as 1:2500
and 1:1250 for country-wide topographic mapping and even
larger scales ranging from 1:240 to 1:1200 for engineering
applications mainly due to operability of
GPS
,
IMU
, and digital
cameras which resulted in nearly complete automation for
various photogrammetric processes, i.e., aerial triangulation,
Digital Terrain Models (
DTM
), orthophoto generation, etc. The
main advantage accrued due to these advancements is that
there is a very little requirement of plan as well as vertical
height ground control points (
GCPs
) for photogrammetric aeri-
al triangulation (
AT
).
It is well established that with aerial imagery flown with
GPS/IMU
for photogrammetric applications,
GCPs
are required
only at the corners of the photogrammetric block. Whereas,
if aerial imagery is flown without
GPS/IMU
, full control points
are required at close intervals along the periphery of the
block, and height control points at much closer intervals in
every flight line.
The provision of control points along with ellipsoidal
heights is an easy task, because these can expeditiously be es-
tablished with the help of
GPS
. But height control points, also
known as Bench Marks (
BMs
), are invariably required with
heights above Mean Sea Level (
MSL
), and therefore, cannot be
determined by
GPS
.
Specifically in countries such as India who have not yet
established their own geoid, the only method available to de-
termine the heights above
MSL
, known as Orthometric heights,
is by precision leveling which is cumbersome, slow, costly,
and error-prone. Some countries like the US and Canada have
established their own precise geoid along with a correction
surface and have made the solution available to the public for
conversion of
GPS
(ellipsoidal) heights to Orthometric heights.
The correction surface is generated from a sufficiently dense
network of
GPS
control points and height
BMs
established by
precise leveling.
The Geomatics Center Canada provides an on-line facility
for direct transformation of
NAD83
or
ITRF
ellipsoidal heights
of any point in the Canadian Territory to
CGVD28
Orthometric
heights, (
CSRS-A
). The National Geodetic Survey (
NOAA
) of the
US has released the
GEOID96
(recently upgraded to
GEOID12A
)
hybrid Height Model for Conversion of
GPS
Heights to
NAVD88
Orthometric elevations (Milbert and Smith, 1996). Facilities
are available online for interactive computation of Orthomet-
ric heights from
GPS
ellipsoidal heights, i.e., the NGS Geodetic
Tool Kit. Because of these resources, these countries are able
to take full advantage of the recent developments and carry
out large-scale photogrammetric mapping most economically
in a much smaller time-frame as compared to others who do
not have such a capability.
India is one among such countries who have not yet es-
tablished a precise geoid along with the required correction
surface. As a result, though India has the capability of acquir-
ing aerial imagery employing most-modern digital cameras
equipped with
GPS/IMU
, the requirement of plan and height
control points for photogrammetric surveys is the same as was
obtaining during the analogue era. The net result is that pho-
togrammetric mapping, even on scale as small as 1:10 000, is
not viable due to its demanding time and cost implications.
An interim solution, especially for such countries, is pro-
posed in this paper to make the large scale photogrammetric
mapping viable which takes max
imu
m possible advantage of
the current advances in the field of Geomatics.
COWI India Pvt. Ltd., 121, Phase 1, Udyog Vihar,
Gurgaon-122016, India, and formerly with the Survey of
India, Hathibarkala Estate, Dehradun-248001, India
(
).
Photogrammetric Engineering & Remote Sensing
Vol. 80, No. 3, March 2014, pp. 253–259.
0099-1112/14/8003–253
© 2014 American Society for Photogrammetry
and Remote Sensing
doi: 10.14358/PERS.80.3.253
