The National Bureau of Surveying and Mapping (NBSM) of the People's Republic of China and the U.S. Geological Survey (USGS) of the United States of America have worked together on joint geographic information systems (GIS), remote sensing, and production management studies and projects. This presentation describes the production of terrain relief maps of China completed under the joint GIS research efforts of the NBSM and the USGS. The relief maps of China at scales of 1:2,500,000 and 1:4,000,000 were completed by the NBSM using techniques developed and tested jointly with the USGS. Black-and-white and color terrain relief maps were produced using NBSM 1:1,000,000-scale digital elevation models and other topographic data, a C language program, and ArcInfo's GRID and TIN modules. The maps can be used for geomorphic, geologic, and geographic analysis and in combination with other thematic data.
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government or the Government of the People's Republic of China.
INTRODUCTION
The National Bureau of Surveying and Mapping (NBSM) in the People's
Republic of China and the U.S. Geological Survey (USGS) in the United
States of America worked together to produce shaded-relief maps of
China. This project is in accordance with Annex II of the Protocol for
Scientific and Technical Cooperation in Surveying and Mapping Studies
between the two national agencies as outlined in the general objectives
covering the period 1991 to 1996. The NBSM and the USGS have
worked together on geographic information systems (GIS), remote
sensing, and production management studies and projects since the
Protocol was established in 1985. This paper describes the methods and
procedures used in creating shaded-relief maps of China using NBSM
1:1,000,000-scale digital elevation model (DEM) data, a
1:1,000,000-scale cartographic data base, a C-language program, and
ArcInfo.
NATIONAL FUNDAMENTAL GEOGRAPHIC INFORMATION
SYSTEM OF CHINA
The National Fundamental Geographic Information System (NFGIS) of
China (formerly known as the National Land Information System) is one
of the largest national GIS data bases in China and a subsystem of the
National Economic Information System of China. It has been under
development at the National Geomatics Center of China, an agency of the
NBSM, since 1984. The NFGIS provides China a common, basic spatial
information system.
The NFGIS is divided into two levels: the National Center System in
Beijing and subsystems in each province. At the national level the data
are at scales of 1:1,000,000, 1:250,000, and 1:50,000. The provincial
subsystems maintain data at scales of 1:250,000, 1:50,000, 1:10,000, and
larger. The National Center System and the provinces work together to
collect and maintain the data at each level.
Several types of data bases make up the NFGIS: topographic, geographic
names, geodetic control, and gravity data. The topographic data base is
at scales of 1:10,000, 1:50,000, 1:250,000, and 1:1,000,000. Plans call
for image data and raster scanned graphic data bases to be included in the
future.
The initial task in developing the NFGIS was to build the
1:1,000,000-scale topographic, geographic names, and DEM data bases
for the entire country and the gravity data base for the southwest part of
the country. Now, the 1:250,000-scale topographic and geographic
names data bases are being built, and the 1:50,000-scale data bases are
being planned.
The main contents of the 1:1,000,000-scale topographic data base are
surveying control points, hydrography, populated places, transportation,
boundaries, hypsography, and vegetation. The data are fully topologically
structured and attribute coded. The data category and attribute codes are
used to describe features represented as a point, line, or area.
DEM data were collected from more than 10,000 topographic map sheets
at scales of 1:50,000 and 1:100,000. About 25,000,000 elevation points
were collected from map sheets at these scales to cover the entire
country. The elevation points are spaced at every 28.125 seconds of
latitude and 18.750 seconds of longitude. The ground resolution ranges
from 800 by 600 meters in the southern latitudes to 450 by 600 meters in
the higher latitudes.
COLLECTING DEM DATA
The NBSM uses several approaches to collect DEM data. For the
1:1,000,000-scale DEM's, data were directly read from paper maps at
scales of 1:50,000 and 1:100,000. Masks consisting of a 32- by 32-line
grid were overlaid and registered on the 1:50,000-scale map sheets.
Elevation values were visually interpreted according to the point's
position relative to the intersection of the grid lines. To avoid
interpretation errors, two different technicians independently interpreted
each point. A computer program checked DEM data points to search for
unusual elevation values. A logical check of the data was done to identify
all points larger than the highest elevation in China. The data were also
displayed to identify and correct errors. The data were grouped and
stored in blocks measuring 3 degrees of latitude by 2 degrees of
longitude.
The vertical elevation error allowed was one-half of the contour interval
of the source map. The DEM's root mean square error (RMSE) for
various terrain is listed in Table 1.
The 1:1,000,000-scale DEM data have supported the realtime monitoring
of floods, research on long-term earthquake forecasting, and planning and
management of radio, television, and FM frequency ranges. The DEM
data were also used to perform slope analysis, define geomorphic zones,
plan agriculture and forest areas, and derive a 1:4,000,000-scale
geomorphic map of China. This map was the first of its type produced in
China using DEM data and digital cartographic production
techniques.
SHADED-RELIEF MAPS
A shaded-relief map is a thematic map that shows geomorphic and
elevation change by using varying shades of gray corresponding to
different slope and aspect values. Before computer technology was
available, relief maps were traditionally hand drawn by skilled
artisan-cartographers, a time-consuming and difficult task. The quality of
the finished map was dependent upon the cartographer's imagination and
artistic skill.
Today, shaded-relief maps can be created by computer-based methods. A
GIS is used, and the relief map is created by mathematical calculation
rather than by the cartographer's eye. Parameters include cell size, light
direction, and viewing angle. The value of the gray scale (V) of each cell
is a function of slope (S) and aspect (A):
V = f ( S x A )
Black-and-white relief maps are very similar to aerial photographs or
satellite image maps, but without cloud or vegetative cover. The gray
values of cells can be changed by using different parameters to enhance
the three-dimensional effect. Cell size can be changed and smoothed to
change the map detail. The black-and-white relief maps of China at the
scales of 1:2,500,000, 1:4,000,000, and 1:6,000,000 serve very well for
showing the characteristics of geomorphic patterns of China, such as the
three major geomorphic regions of the country, the Loess Plateau, the
Sichuan Basin, and the desert regions. The relief map could be used for
geomorphic analysis and is a good background for certain kinds of
thematic maps such as land cover and land use, weather, and geologic
structures.
CONVERTING THE DEM DATA
Converting the DEM data into a useable format was the first step in
compiling a shaded-relief map. It was necessary to convert and resample
the data for import into ArcInfo's GRID module, to reduce the
number of data points from 25,000,000 to a more manageable number,
especially since all the points were not necessary for display at small
scales. Because the data are collected and stored in 3- by 2-degree
blocks, the tiles need to be mosaicked together to form a continuous
coverage. Two DEM conversion methods were developed and
tested.
The first method used ArcInfo's TIN module to convert, resample,
and mosaic the DEM data. The method was tested using a 6- by
4-degree test area consisting of four 3- by 2-degree blocks of data. This
entailed creating an ARC point coverage with the DEM data points and
converting the file into a GRID file using the TIN module functions of
CREATETIN and TINLATTICE.
The first step in this process was to rewrite each of the 3- by 2-degree
blocks of DEM data into a new file readable by ArcInfo's
GENERATE command and the format:
elevation value longitude latitude
This step also was used to mosaic four of the 3- by 2-degree blocks
together into a block of 6- by 4-degrees. The next step created an
ArcInfo point coverage using the data from the previous process.
The data were then projected into the Lambert projection commonly used
in China to display maps of the entire country. The next step was to
convert the point coverage into a triangular irregular network (TIN)
format. The last step resampled the TIN file into cells with a ground
resolution of 1,500 by 1,500 meters. Although this method produced
data at the appropriate level, the processing time for the test area was too
long to use to process data for the entire country.
In the second method, a C language program running under the UNIX
operating system resampled and converted the data. The first step read
and mosaicked all the DEM data and projected them onto a Lambert
projection. The data were resampled using a weighted mean square
distance algorithm (see figure 1).
Where Z is the new elevation point calculated from n number of elevation
points spaced a distance of d from each other.
In the next step the data were converted into an ArcInfo ASCII
format from which a GRID file was created. The process successfully
created an ARC GRID that could be used to generate the shaded-relief
map.
Both methods have advantages and disadvantages. The first method
tested uses only ArcInfo, which eliminates the need to write and
compile a separate program. The processing is very time consuming and
the interpolation formulas are limited to those available in ArcInfo.
The second method is more flexible because the interpolation method was
written specifically for the desired result. The processing speed is faster
and more efficient in the second method than in the first. The second
method was selected to complete this project.
CELL SIZE
The cell size of the 1:1,000,000-scale DEM varies depending on the
latitude. Since the elevation points are spaced every 28.125 seconds of
latitude and 18.750 seconds of longitude, the ground resolution ranges in
size from 800 by 600 meters to 450 by 600 meters. The appropriate cell
size used on a given map depends on several elements, such as the
original size and accuracy of the DEM, the geomorphic shapes of the area
displayed, map scale, and software limits. Generally, depending on the
scale, the smaller the cell size is, the better the results are. Nevertheless,
smaller cell sizes can result in large data volumes and long processing
times. The cell size should be appropriate for the map scale. Normally,
on the output product, the cell size of 0.1 to 0.25 millimeters (mm) is
appropriate for small-scale, page-sized maps and 0.3 to 0.5 mm is
appropriate for small-scale wall maps.
Two black-and-white shaded-relief maps were prepared as part of this
project at scales of 1:4,000,000 and 1:6,000,000. A cell size of 0.38 mm
was used for the 1:4,000,000-scale map and a cell size of 0.25 mm for the
1:6,000,000-scale map. This corresponds to a ground resolution of 1,500
meters.
LIGHT DIRECTION AND ALTITUDE ANGLE
Shaded-relief maps can show light coming from any angle; two
parameters used include direction and altitude. To get satisfactory
three-dimensional visual effects, parameters were selected that do not
match the parameters for true sunlight. For this project sunlight came
from the northwest at an azimuth of 315 degrees (figure 2).
The light angle is another parameter used to enhance the
three-dimensional effect of the map. In this project, 45 degrees of
altitude was used.
VERTICAL EXAGGERATION
Vertical exaggeration means that the elevation value of each cell is
multiplied a certain amount so that the changes in the vertical direction
become more evident. For displaying small-scale maps or flat terrain, it is
useful to exaggerate the elevation values. But, the higher the vertical
exaggeration is, the longer the shadow will be when using a low-altitude
angle for sunlight. This can cause the adjacent geomorphic shape to be
hidden within the shadow of a larger feature. In this study, a vertical
exaggeration of 5 was chosen. The azimuth, altitude angle, and vertical
exaggeration parameters need to be adjusted to get the best visual effect
for each output product.
MAKING THE BLACK-AND-WHITE RELIEF MAPS
The contents of the black-and-white relief maps of China at scales of
1:4,000,000 and 1:6,000,000 are created from the 1:1,000,000-scale
DEM and vector features, including the major rivers, roads, railways,
large cities, names, and boundaries selected from the 1:1,000,000-scale
Cartographic Data Base of the NFGIS.
The ArcInfo ARCPLOT module was used for designing the map
symbols, colors, and structure. The first step was to create the shade file
from a GRID file. Interactive editing can be done if necessary. The
shade file was cut using the Chinese national boundary. In the next step,
selected features from the 1:1,000,000-scale Cartographic Data Base of
the NFGIS were transformed into the same projection with the surface
shade file. The features selected were national, provincial, and county
boundaries and names, coastline, rivers, railways, and major roads.
Symbols, legend, illustrations, and map names were then designed using
ARCPLOT. An ARC Macro Language (AML) program was used to
overlay and register the raster and vector data and create the plot file. A
black-and-white version of the map was produced using a Calcomp
electrostatic plotter (see figure 3 for an example).
CREATING COLOR RELIEF MAPS
Based on black-and-white relief maps, the color relief maps were created
by adding color to each cell according to its elevation. The original gray
values of the black-and-white relief maps were used to assign value. Hue
and saturation of each cell were calculated with a color look-up table
according to the cell's elevation value. The three factors, value, hue, and
saturation, for each cell were used to create different colors. The DEM
processing and plot generation procedures used to produce a color relief
map were similar to those used to produce a black-and-white relief
map.
The principles of the color look-up table for the color design were as
follows:
1. The higher the elevation value of each cell is, the darker the color to
be used.
2. Color will change smoothly from blue to green, to yellow, to orange,
and to brown as the elevation changes from sea level to plain, hills, and
mountains. Between every two adjacent colors, there is a gradual
transition of one color to another to make the color change
continuous.
The color relief maps of China at scales of 1:2,000,000, 1:4,000,000, and
1:6,000,000 were also overlaid with vector features, such as boundaries,
roads, railways, major rivers, cities and counties, and geographic names.
These maps have similar characteristics to the black-and-white relief
maps. In addition, it is easier to understand the terrain characteristics
when using color.
CONCLUSION
The 1:1,000,000-scale DEM of the NFGIS can be used in many ways,
ranging from analytical studies to the production of a shaded-relief map
of the country. Methods that were researched, developed, and tested
were used to produce shaded-relief maps of China. This research serves
as an example of the benefits of the cooperative research on GIS
technology between China and the United States.