LESS MEANS MORE – NTDB AT SCALE 1:100 000
Since the very beginning of our childhood we always wished for more and
more of many things. We wished more happiness, success,
money, satisfaction and so on. Most of the times, we are
satisfied if we have more of the things we are looking for. A
child is much happy if he has more and more toys to play
with. A student waiting for the examination result is wishing
more and more marks in each and every subject. An employee
naturally wishes for more and more salary from his job. Well,
this can be continued for more and more cases. There are
things which we always want less and less if not nil. A child
naturally wants less and less comments from his guardians, a student
wishes for less and less homework and so on. Working in the
field of mapping and geospatial data, let us think, are there anything,
which we want less but at the same time we get much more?
While making maps or creating geospatial data, we always tend to
include more and more information as far as possible. But how
much of this “more information” is sufficient to meet our
desire? Practically, a map or a data storing mechanism has a
certain limit to the volume of information it can hold. The
first of such limits is imposed by us, by defining the use of a map or a
data set. We tend to include that information, which play
role in fulfilling our requirements. We can see this clearly
in case of most of the thematic maps. Another limiting factor
is the space available for showing the information. In case
of hard copy maps, the space available is controlled by the map scale
and in case of digital data, it is controlled by the space available in
the data storage devices. Similarly, the ability of human
eyes to discern two or more objects lying side by side also controls the
possible volume of information shown in a map.
The information content of base maps is very much
critical. By the very definition, a base map should offer as
much details as possible. But at the same time we should not
forget about the information noise in a map or data. In case
of digital data, the volume of information plays a vital role in further
processing of the data. Most of the software packages
available in market, working with digital data, have their limit in
processing the data. Huge volume of data can’t be, at least
very much time consuming, processed due to limitations of hardware and
software packages.
The question “How much information should be shown in a map or data?”
is still unanswered. Traditionally, in different countries
different volumes of information are shown in maps of different
scales. The information content of maps often reveals the
economic and technological achievements of a country.
In Nepal, after a gap of about 50 years, a new series of base maps have been
produced during 1990s. Before the publication of these maps,
the map users were compelled to use “one inch to a mile” topographical
maps published by the Survey of India. The development in the
field of digital mapping and geographical information system created a
huge demand of digital geospatial data. Being the national
mapping agency, Survey Department took the responsibility of fulfilling
this growing demand of the map users. The will power of
Survey Department of Nepal made it possible to convert all those base
maps into the digital form and the digital data are made available to
the users.
The feedback from the users revealed some inconveniences in using the
data. The data file, a set of individual layers like
boundaries, buildings, hydrographic features, landcover features etc.,
has been managed in the basis of a map sheet creating problems of edge
matching between the data files. The projection and
coordinate system of data file is same as that of the corresponding map
sheet creating problem in displaying the data for most of the data
users. This is because of the projection and coordinate
system used in the preparation of those maps. For the mapping
purpose modified Universal Transverse Mercator Projection (MUTM) is used
in Nepal. To keep the deformations within certain limit, the whole
area of Nepal has been divided into three 3° zones. For each zone,
the intersection of the central meridian and the equator has been
considered as the origin for the coordinates with false northing of 0 m.
and false easting of 500 000 m. assigned to it. So, for the
proper viewing of the data in computer screen, all data should first be
converted into single coordinate system.
The data files often contain error prone features in
them. During the editing of the data, some features were
intentionally introduced to make the processing of the data
possible. This is because of the limitation of the software
(PC ARCINFO Ver. 3.4.2) used for the creation of the data file.
Survey Department has always done it’s best to serve the map users and
solve their problems in the field of mapping. Continuing this
culture, Survey Department is working on creating data files at smaller
scales. It has plan to create data files at scales 1:250 000,
1:500 000 and 1:1000 000. The data files at scale 1:100 000
are already available for the users.
Creating data files at smaller scales means generalizing the data
content of larger scale data files. In essence, this means to
reduce the number of features in the data files and cover larger ground
area in the data file i.e. achieving more from the
less. According to the map sheet layout in practice in Nepal,
a data file at scale 1:100 000 covers the ground area covered by 16 data
files at scale 1:25 000 or 4 data files at scale 1:50
000. This greatly reduces the problems of edge
matching.
Due to technological and manpower constrains, not all aspects of
cartographic generalization have been implemented in the data
files. Most frequently applied aspect of generalization in
creating data files at smaller scales is the selection of
features. For example, several building clusters within a
specified area defined by a circle of specific radius are represented by
a single point. Different types of points present in the data
file of buildings viz. residential buildings, religious buildings and
other types of buildings are processed separately. Rivers of
order 1 are removed if their lengths are less than specific
length. Landcover polygons with area less than a specific
area are dissolved into neighboring polygon of same category with
largest area. Selected category of transportation lines is
shown. Other aspects of cartographic generalization can be
applied in future as new software packages, capable of performing
cartographic generalization, become available. For example,
line smoothing, making interrelated features consistent, displace
transportation lines in accordance to the contour lines, and rearrange
administrative boundaries as per the generalized river lines and so
on. Similarly, availability of additional attribute
information of the features present in the data file can assist in
generalizing them in more scientific way. For example,
population by settlements may help us in ranking the settlements which
may be the input for generalization of settlements.
The data files at scale 1:100 000 are stored in geographical coordinate
system. This will solve the problem of displaying the data for
most of the data users. Advanced users can display / convert the
data file into a projection satisfying their needs. The Everest
1830 Spheroid is used in Nepal for mapping purposes. The
major, a and minor, b semi axes radii are 6 377 276.345 m. and 6
356 075.413 m. respectively. The data files at scale 1:100 000 are
in ArcView shape file format, which most users are familiar with.
This format can be converted into most of the other spatial data
formats.
The users interested to acquire the data should write an official
letter explaining purpose of data, required sheet numbers, required
layers, name and official position of the person who will receive the
data and sign the agreement.
The letter should be addressed to The Section Chief, Digital Mapping
Unit, NGIIP, Survey Department, Minbhawan, Kathmandu.
Comments
Post a Comment