PI-7-3 
3. ALGORITHM 
Drainage Direction(flow direction) is set as the 
steepest descent direction among the neighboring 8 
direction by using DEM. The problem is that the 
drainage direction can not be set at the concave pixel 
or flat area as mentioned in above. In order to solve the 
problem at concave pixel, a method increasing the 
altitude of the concave point to make over-flow to 
down stream is suggested and it is well 
known(Nogami,1998). And for flat area problem, 
some methods are reported to give certain drainage 
direction for such pixels to reach to bottom of the 
stream. In our experiences, if the target area covers 
within a few sheet of 1:50,000 scale map and pixel 
resolution is not more than 100m x 100m, the 
automated extracted river system are very similar to 
the reliable river system information, except Oat area 
where the auto-extracted river line has a controlled 
direction to the down stream, and the streams do not 
run along the reliable stream lines in many cases. 
In this study, global DDM is produced with 30 seconds 
arc grid( about 1km resolution at equator) for lands. At 
first, DDM was produced by using only GTOPO30, 
correcting concave and flat area. It is confirmed that 
most of computer extracted streamlines run in the 
bottom of topography(valley) when it overlay with 
shading image derived from DEM, and most of them 
run along the streamlines of DCW and WBDb-II data 
sets within several pixels distortion. However some of 
the computer extracted streamlines have following 
problems and the problems make the accuracy of the 
DDM quite bad in some watersheds. 
(1) For example, where three rivers - Yangzhu river, 
Mekong river and Tanlwin river - run closely at south 
of China, the computer extracted rivers make wrong 
watersheds due to the confused DDM. It was easy to 
detect such wrong river systems because the wrong 
watersheds are enough big to detect. However similar 
confusion must be happening in ¿mailer watersheds, 
Fig.-l Extracted Watersheds comparing DCW data