1. Finding Craters Under a Dense Forest
In November 2003, laser measurements were taken from an aircraft over the summit area of Hachimantai [2]. By removing the effect of trees, researchers created elevation data with a 1-meter interval. When they made a red relief map from this data, two spots in the dense forest appeared to have clusters of small craters [2].
Researchers walked to the locations found on the map to check them in person. They confirmed 13 bowl-shaped depressions at four sites, and another 16 near the Gozaisho area [2]. It is thought that these are likely craters rather than landslides [2]. In a forest, you cannot see the ground from a normal photo. Yet, the map showed the bumps and dips under the trees. How was the height of the ground measured?
2. Lasers Pass Through Gaps in the Leaves
A laser scanner on an aircraft sends pulses of light toward the ground. The distance is calculated from the time it takes for the light to bounce back [7]. This distance is combined with the aircraft’s position, known from GPS, to find the exact 3D location of the ground [7].
In forests, it is said that some laser pulses slip through gaps between leaves and hit the ground [8]. Computers remove signals that bounced off tree tops, leaving only the data from the ground [8]. This creates a "bare earth" model without trees. The Geospatial Information Authority of Japan started this laser surveying in 2002, recording elevation every 5 meters in units of about 10 cm [3]. Because it measures the ground directly, high precision and density are said to be obtained [3]. Now the ground height is a list of numbers, but numbers alone do not show the shape of the mountain. What kind of map is needed?
3. Light Direction Changes What You See
Before red relief maps, other maps tried to show terrain, but each had weaknesses [1]. Contour lines are hard to use for both gentle and steep slopes. Slope maps show steepness with color but cannot distinguish ridges from valleys. Elevation color maps cannot show small landforms [1].
Shaded relief maps mimic light hitting the land from one direction. They look 3D, but the direction of the light limits which features are visible [1]. Can we draw a map that shows ridges and valleys without deciding on a light direction?
4. Red Shows Slope, Brightness Shows Shape
The red relief map combines two images. The first image shows slope steepness as the intensity of red [1]. The second image shows the degree of ridge or valley as brightness [1]. Combining them in color creates the red relief map [1].
The second image uses a value called "ridge-valley degree." This comes from "sky view factor," which averages the angle of the horizon from the zenith in 8 directions [1]. Ridges and isolated peaks have high values; valleys have low values [1]. If you flip the terrain upside down and do the same calculation, you get "ground view factor," which highlights valleys [1]. Subtracting the ground view factor from the sky view factor and dividing by 2 gives the ridge-valley degree [1]. Ridges are positive, valleys are negative, and flat areas are near zero. This brightness image is like ambient light in computer graphics and does not depend on light direction [1]. The two methods cover each other’s weaknesses: slope maps miss ridge/valley distinction, and ridge-valley maps miss steepness. Together, they fill the gaps [1].
5. The Map Points, Humans Check the Ground
Features found on a map are only "likely" until checked. In several bowl-shaped depressions near Gozaisho, researchers found abnormal ground temperatures, hydrogen sulfide in spring water, and the mineral limonite at the bottom of the depressions [2]. Based on these findings, it was concluded that they are likely craters, not landslides [2]. However, the actual erupted material has not been confirmed, so the report says "craters are the most likely possibility" [2]. The map did not give the final answer; it pointed to places to investigate. It served as a tool to find terrain hidden under trees.
6. Born from Mt. Fuji Disaster Maps, Now Nationwide
The red relief map was invented by Tatsuro Chiba of Asia Air Survey Co. [1]. Chiba and Mr. Suzuki presented the method in 2004 and got a patent in 2005 [1]. It is said that the idea came from making disaster prevention maps for Mt. Fuji [5]. When laser measurements were done in the Aokigahara forest, a different ground surface appeared compared to maps made from aerial photos [5].
On June 6, 2018, the Geospatial Information Authority of Japan released red relief maps for all of Japan on the "GSI Map" [4]. These were made using the Asia Air Survey patent and 10-meter mesh data [4]. The Hachimantai data used elevation data at 1-meter intervals, while the national map uses 10-meter mesh [2][4]. A 10-meter square contains 100 1-meter squares [4]. Finer grids are better for finding small depressions.
7. Try It: Crumpled Paper and a Light
Crumple a piece of paper with your hands, then spread it out on a desk. Shine a light from the side and look at the bumps and dips. Move the light to the other side. The shadows change, and some features become harder to see. Compare this to looking at the paper with just room light [1]. This small experiment shows the weakness of shaded relief maps: the direction of light changes what you see [1]. Do not shine the light into your eyes.
You can view the national red relief maps on the GSI Map website [4]. Look at your town or a favorite mountain to see how ridges and valleys are drawn in red and brightness. The method is summarized in the 2007 report by Chiba et al. [1], and the Hachimantai crater story is in the 2005 report by Wachi et al. [2]. Note that the origin story is based on news reports, and verifying it with primary sources is a future task [5].