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It has always been an important low-level operation to extract edges from images in the fields of computer vision and
image procession, in which straight line extraction is typical and representative. Because most man-made spatial objects,
e.g. buildings, roads, etc. often take on near straight-line boundaries, extracting straight lines is often the first step to
extract these targets. Straight lines can then be looked as the elementary units for other higher level image
interpretations. In this paper, a straight line extraction method combining edge detection and depth-first searching on the
vector line layer is proposed and applied to extract runways of airports. The steps include: 1) edges are found with the
Canny operator and vectorirzed. The reason to use the Canny operator is because it is designed to be an optimal edge
detector, which gives very good results on detecting step or slop like edges. It takes as input a grey scale image, and
produces as output an image showing the positions of tracked intensity discontinuities. After this operation, we then
vectorize the edge points to be a vector layer with edge tracing.2) With the vector-formatted edge lines, the straight line
searching can then be carried out. In order to complete this, topology between arcs should be cleaned and rebuilt, which
includes the deletion of repetitive, one-node arcs, and splitting on the intersections, etc. 3) Straight lines are detected with
the depth-first searching strategy. With the rebuilt topology, we can easily obtain the begin, end nodes of every line. If
the distances of its all vertices to the line connecting the begin, end nodes of an arc are less than some pre-defined
threshold, it could be looked as a 'straight line' and extracted. Besides, we are certainly only interested in the straight
lines with lengths larger than certain threshold, thus a minimum length threshold should be specified to delete these very
short lines. In the searching of straight lines, some arcs should be grouped as a single straight line; some un-straight lines
should be split to extract its straight parts. The suitable straight lines are outputted to a vector layer after being reselected
and re-grouped, with distinguishing short, long isolating, long not isolating straight lines. With all these steps,
we can get the initial straight vector line layer. 4) To these lines with small interspaces but locate on a single straight line,
we use a simple but effective connecting step to 'fill' the gaps. Starting from the vector layer and with the operations of
broken line connecting and parallel line detection, the main airport runway can be well extracted, which helps us to
locate and recognize airports from high spatial remotely sensed imagery.
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