G.P. and R.J. Pike, 1991, Pike R.J. and G.P. Thelin, 
1992). Digital shading using DEMs, gives the possibility 
of creating realistic, detailed and accurate images of the 
relief (Burrough 1986, Thelin G.P. and R.J. Pike, 1991). 
The brightness of each point or pixel in the shaded relief 
image, depends on its elevation relative to the elevation 
of its neighbors, its slope and aspect, and the simulated 
position of the source of light; direction (azimuth) and 
elevation (angle above the horizon). 
Shades are a primary clue to depth perception. But true 
3D peception involves the existence of two images of the 
same place or feature seen from different angles (Toutin, 
P. And B. Rivard, 1995). A virtual 3D image of the 
shaded relief can be generated by applying the DEM to 
the original shaded relief image to obtain a second image 
with parallax. For viewing, one stereoscopic image is 
made assigning the original image with the blue and 
green colors, and the image with parallax is assigned the 
red color. When seen through anaglyphs a 3D image 
"jumps". Other techniques include the use of polarized 
glasses, stereoscopes, etc. 
In a new technique, Toutin, T. and B. Rivard, 1995, use a 
single color image taking advantage of the effect of 
chromostereoscopy. In this case, depth is coded in color, 
so that blue hues are seen farther, and red objects look 
closer. A hypsographic map, when seen through special 
lenses, the areas in hues of blue and green will seem to 
be at lower elevations, and those with red will appear 
higher. 
DATA AND METHODS. 
Digital elevation models and vectorial data. 
The 255 DEMs that cover continental Mexico and most of 
its islands were used. Taking as a base the 1:250,000 
scale topographic map series from INEGI, these models 
were generated by the United States Defense Mapping 
Agency (US-DMA), and currently are available to the 
general public. Vectors for international borders and 
coastlines were obtained from the 1:4,000,000 scale 
topographic map from DGG-INEGI. 
Digital elevation model processing. 
The individual one by one degree models were combined 
first to form larger models (6 degrees of longitude by 4 
degrees of latitude), keeping the resolution (3 seconds) 
and geographic projection. A single elevation model was 
designed to cover all the Country, with elevation values 
every 500 meters and in a Lambert Conical Conformal 
projection. Using the EASI/PACE* program REGPRO 
running in VAX workstations, all the mosaics were 
registered to this single model. The registration process 
involved a resampling of the elevation data and a 
projection change. No estimation of the accuracy of the 
resulting model was made. 
Shaded relief image. 
The shaded relief image was obtained from the DEM 
using the program REL, with 315° (NW) of azimuth and 
* Mention of brands or companies is for information purposes only. 
  
630 
an elevation of 30°. A mask was created for all the zero 
elevations, corresponding to the sea, and to portions of 
the model with no data (other countries). The image 
under this mask was set to a gray value of zero to 
separate them from flat areas above sea level. The 
shaded image was transformed through a piece-wise 
transformation to enhance the relief perception in 
relatively smooth areas. 
Stereo shaded relief image. 
Taking the first image and combining it with the DEM, a 
second image with parallax was generated using the 
program STER. The factor for parallax was 100. This 
means that the displacement in number of pixels for a 
given point between the original image and the second is 
the elevation value from the DEM divided by 100. 
Shaded hypsographic image. 
The elevations were divided in 11 ranges, most of them in 
500m intervals. For each range, a mask was created 
from the DEM. Next, one base color was selected for 
each interval, going from light blue in the sea (elevation 
zero), and blue-green in the lowest parts to dark brown 
and gray in the highest points. The colors were first 
selected in a Red Green Blue (RGB) color space, and 
then were mapped to the masks corresponding to each 
elevation range. The three band RGB image was 
transformed to an Intensity Hue Saturation (IHS) image. 
A lookup table for transforming the shaded relief gray 
image, was prepared for each elevation range. In the 
lookup tables the value 128 (mean value in the gray 
image) was substituted by the intensity value of the color 
of the corresponding interval. Besides, the lookup tables 
included a piece-wise stretching to enhance relief 
perception. The lookup tables were then applied to the 
shaded relief image to derive a new image, which was 
then used as the intensity channel. The IHS image was 
transformed back to RGB. This resulted in an image in 
which each elevation range has a single base color 
(same hue and saturation) but different shades (intensity) 
of the same color, lighter of darker, according to the 
shaded relief. A series of tests with different colors was 
conducted until a satisfactory color combination was 
achieved. In the last trials the colors were chosen directly 
in the IHS color space; keeping constant the saturation 
level, and varying the intensity and hue. This was also a 
better way to make sure that a gradient from colors with a 
major blue component to those with red as the main 
component was produced. 
Map production. 
The previous paragraphs describe the DEM and image 
processing. For the final map production, the images 
were exported to the Laser - Scan (LAMPS) environment, 
for adding a frame, international borders, names, 
graticule of meridians, parallels, and legend. The final 
steps were made with Barco Graphics software, and 
included the transformation of the RGB images to CMYK 
(for the 3D and hypsographic maps) the integration of 
images, with the cartographic frame, preparation of hard 
copy tests and print positives, with color separates in the 
case of the 3D and the hypsographic maps. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996