Need for Precision Products
Definitive orbit and attitude information are used for standard
products generation. The overall accuracy of the standard products
comes to + 1.5 km. For the digital database generation in GIS
environment the most significant aspect lies in the geometric
accuracy. The information derived from different sources should
have geometric compatibility to establish one-to-one
correspondence. This will facilitate overlaying, merging, integrating
various information and form the basis for further analysis. The
inherent error in the standard product unless corrected will create
problems in seamless integration of digital database and in turn
generation of precise developmental action plans.
Table No. 1 Error Budget Analysis in Generating Precision Products
| S.No | Parameter | Digitizer based | Scanner based | GPS based |
| 1 | Inherent map accuracy | 12.5 | 12.5 | - |
| 2 | Projection error | 2.0 | 2.0 | 2.0 |
| 3 | Map-digitizer transformation error | 25.0 | - | - |
| 4 | Image-map transformation error | 30.0 | 15.0 | - |
| 5 | Error in the tiling of reference maps | - | 6.0 | - |
| 6 | GPS receiver positioning at the GCP | - | - | 2.0 |
| 7 | Baseline accuracy | - | - | 1.0 |
| 8 | Image-GCP transformation error | - | - | 12.0 |
| | Total error in meters (Approximate) | 65 – 75 | 35 – 45 | 15 – 20 |
Methods of Generating Precision Products
The desired accuracy required at 1:50000 scale may be of the order
of 1mm of the map units i.e. less than 50 meters on ground. Using
precise ground control points, the desired geometric accuracy of the
satellite data products can be achieved. The number and distribution
of ground control points and the order of transformation model will
influence the accuracy of the geometric correction.
The three basic methods adopted for acquiring GCPs are
Digitizer based Method, Scanner Based Method and GPS Based
Method. Digitizer based method is a two-step approach. At the first
level map-digitizer model is established and this will facilitate
transformation of digitizer coordinates to map coordinates. At the
second level, image-map model is established to facilitate one-to-one
correspondence between image and ground coordinates.
Similarly, scanner based method is also a two step approach.
At the first level map-gratic model is established and this will facilitate
tiling of maps with respect to well defined graticules. At the second
level, image-map model is established to facilitate one-to-one
correspondence between image and ground coordinates.
In contrast to the above two methods, GPS based method is a
single step approach, wherein a transformation model is established
between the post processed coordinates of the GCPs and image
coordinates.
Study Area & Data used
The study area is located in and around Bhisi, Chimur Taluk,
Chandrapur district, Maharashtra. The study area is 70 km away
from Nagpur and lies in between 79 0 15’ and 79 0 30’ Eastern
longitudes and 20 0 30’ and 20 0 45’ Northern latitudes. The IRS 1C
LISS III (100-58) and PAN (100-58-A) data of the corresponding
area are used for generating precision products. The corresponding
reference maps of 55P/6 on 1:50000 and 1:25000 scale are also
used. Single frequency geodetic GPS receivers are used for GPS observations.
Methodology
The three methods discussed above are used to generate precision
products. In the first method 1:25000 reference maps are mounted
on the digitizer one at a time and individual map-digitizer models
are established. All the graticule intersections are used as reference
points and a second order transformation model is established. This
serves as a basis for establishing image-map transformation model
using well-defined and well-distributed GCPs
In the second method, the reference maps are scanned and
tiled with reference to the predefined
graticule base using a transformation
model. This serves as a basis for
establishing image-map transformation
model using well defined and well
distributed GCPs With the advent of
scanner technology, this method has
become more popular and widely used
globally.
In the method using GPS, the first
task will be establishment of a reference
point with accurate coordinates. A
permanent monument is constructed with
well-defined marking on a brass plate.
The ornamentation point is observed for
a continuous 72 hours using dual frequency geodetic GPS receiver.
Using Bernese software, the observations are processed in differential
mode using post-processing data. The precise coordinate of the
ornamentation point is arrived at with reference to the five
permanent stations of WGS 84 datum with long baselines of the
order of 2000 kms. The accuracy achieved for this coordinate is of
the order of 0.02 ppm.
Around 16 well-defined and well-distributed GCPs are identified
on the image as well as on the map. Reconnaissance survey is carried
out to identify the local reference station. The study area is around
70 km away from this reference station. A reference point is
established in the centre of the study area by taking continuous
observations for a period of eight hours in differential mode with
reference to monumenation point.
The baseline vector is computed and the absolute coordinates
of the reference point are arrived at. At each of the GCP locations,
2-hour GPS observation is carried out. The GPS data is post processed
using SKI L1 software in differential mode to arrive at the absolute
coordinates of the GCPs.
A transformation model is established between these
coordinates and the corresponding image coordinates. Using this
model, satellite data is georeferenced.