Hi mamamary,
When your son's teacher asks "How were topographic maps drawn 20 years
ago and how are they drawn today", I don't believe the teacher is
referring to the appearance of the finished map. Instead, the teacher
is likely to be referring to a profound change in the way that the
master copy of the map is produced by the mapmaker.
Let's first look at how topographic maps were made twenty years ago.
First, the area is surveyed to gather the information that will make
up the map. Aerial photographs are taken of the area to be mapped.
Each part of the land is photographed at least twice, from different
points on the flight path. Adjoining pairs of aerial photographs can
be placed into a stereoscopic viewer which produces a 3-dimensional
effect that enables the mapmaker to see the height of the ground at
each point.
Aerial photography for mapmaking was itself a revolutionary change,
but it was first introduced in the 1930s and was solidly established
twenty years ago.
The mapmaker will also need survey data gathered from the ground.
Selected points that fall within the map area are visited, and angles
are carefully measured from those points to other points whose height
and position are accurately known. From these measurements,
co-ordinates can be calculated that provide a precise framework onto
which the aerial photography can be projected.
Next, the 1984 mapmaker creates the master copy of the map from which
all others will be reproduced. The stereo aerial photos are mounted in
a projector, adjusted so that the ground-survey points are projected
onto the right position on the map manuscript. The mapmaker then
traces the position of the map features, creating the master map.
As the mapmaker looks through the 3-D glasses, the projection can be
adjusted to identify land that is at a certain altitude. The mapmaker
then traces these points as a contour line, before adjusting the
projection to trace the next contour line, and so on.
Next, the basic map data is duplicated onto layers of film - one for
each color in the final printed map. Again, the aerial photographs are
projected. For each piece of film, the mapmaker traces the location of
features that are to be printed in that color. For example, areas of
forest will be marked onto the film that will be printed in green;
roads onto the film for the black print; rivers onto the film for the
blue print, and so on. Text and gridlines are also applied to the film
for the relevant color.
Printing plates are then made from the individual films, and a
multi-colour map is produced by printing each of the colors in turn
onto a sheet of paper.
Now we fast-forward to 2004. Aerial photography is still used, as are
ground surveys although the position of ground features is now
obtained from satellite measurements rather than by measuring angles
to visible features.
The biggest difference, though, is that instead of scratching map
details onto sheets of film, an operator examines the aerial photos
and enters the information into a computer file. For example, a river
would be recorded as a list of co-ordinates joining all the points
along which the river flows, a forest would be recorded as a list of
co-ordinates joining all the points along its boundary, and so on.
When it is time to make the map, a computer program reads the data and
generates a file that can be used on a phototypesetting machine to
produce the printing masters. Not all of the data is used for every
map. For example, one map may exclude minor features because of its
small scale, whilst a larger-scale map may include more features. Maps
produced for use in mining might include geological information that
would be undesirable on a road map.
Once the survey data is held on a computer file, the software can
select and assemble whatever is required for the map that has been
specified. The software can even decide where to position names and
other text to provide optimum readability in the finished product.
The task of manually drawing the master copy of the map has been
eliminated. If an error is discovered, it is straightforward to
correct the computer data and run off another print master, whereas 20
years ago it would have been necessary to carefully scratch out the
old data and paste the new data onto the film, or even to redraw the
map entirely for major changes.
Mapmaking is a fascinating subject, and there is much more that can be
said, but I'm not sure how much detail you need.
There is an excellent webpage from the USGS that provides more
information about the old and new methods of mapmaking, and should be
quite readable for a fifth-grade child:
"Topographic Mapping"
http://erg.usgs.gov/isb/pubs/booklets/topo/topo.html
For a broad overview of maps and mapmaking, there is a good article
from the online Encarta encyclopedia:
MSN Encarta - MAP
http://encarta.msn.com/encyclopedia_761577953_2/Map.html
I trust this provides the information you require, but if you need any
further specific information feel free to request clarification.
Google search strategy:
mapmaking traditional topographic
://www.google.com/search?q=mapmaking+traditional+topographic
Regards,
eiffel-ga |
Clarification of Answer by
eiffel-ga
on
12 Jan 2004 13:07 PST
Hi mamamary,
Firstly, please be reassured that you will only be charged one fee (of
$25 plus the 50-cent listing charge) which is for the main answer. Any
material that researchers or the general public post to the "Comments"
section is not charged for.
Secondly, I do recommend that you and your son read the USGS page on
Topographic Mapping that I listed in the main answer. Even if some of
the text on that page is too complex, you may still gain some benefit
from the general overview that it offers.
In addition, I am rewriting my answer in a simpler form, in the hope
that this will make it more useful to you and your son. Good luck with
the project, and if you have any more questions please feel free to
request further clarification.
TOPOGRAPHIC MAPS - 20 YEARS AGO
A plane flies over the area to be mapped, taking a series of photos.
These photos can be loaded into a 3-D viewer to show the height of the
land at each point.
Also, some points on the ground are visited, and a surveyor measures
the angle and distance from those points to other points whose
location is known, such as distant hills. These points provide the
"framework" to which the aerial photos can be matched.
To make the "master copy" of the map, the mapmaker looks through the
3-D viewer and traces lines that join every point of the land that is
at the same height - these become the contour lines on the finished
map.
The mapmaker will then trace additional master sheets, one for each
other color on the finished map. The "blue" sheet would include the
rivers, and you can imagine a mapmaker tracing the paths of rivers
from the aerial photo onto this sheet. The mapmaker would draw areas
of forest onto the "green" sheet, and would add lettering and grid
lines onto the "black" sheet.
From these master sheets a printer will print each color in turn onto
a sheet of paper, which becomes the finished map.
HOW TOPOGRAPHIC MAPS ARE MADE TODAY
Aerial photography is still used, and a surveyor still visits selected
points on the ground. But it is no longer necessary to measure angles
to distant hills. Instead, a radio signal from an orbiting satellite
can tell the surveyor his exact location.
There's no longer any need to draw the map details onto the master
maps. In fact, there's no traditional "drawing" any more.
The mapmaker uses a device that works like a computer mouse with a
pointer attached. The mapmaker simply traces out the paths of rivers,
forests and so on, and the computer stores this information in a data
file.
From this data file, the computer can assemble and print out a map at
any time. What's more, the same data can be used to produce different
kinds of map, such as maps at different scales or maps that include or
exclude certain kinds of information. For example, a road map may not
need to include the height contours.
SUMMARY
20 years ago, the mapmaker had to carefully draw the master copy of
each map. Nowadays, the mapmaker traces the information into the
computer, and the computer can print the map on demand.
Regards,
eiffel-ga
|
