ISPRS, Voi.34, Part 2W2, “Dynamic and Multi-Dimensional GIS", Bangkok, May 23-25, 2001
CARIS (Lee, 1996), for example, a digital photo of a specific
building (the City Hall) can be retrieved as extended attributes by
a map query. Figure 1 is one of the digital photographs of a
wooden house employed by the author for the evaluation of PM
Pro. A total of five digital photographs of this wooden house were
taken with a Fujix DS-100 digital camera. Once the digital
images are processed with proper DCRP software, many more
functions can be accomplished in addition to the simple viewing
of the original image, as will be introduced below, b. Viewing
the ortho-photo: As a 2D perspective projection of the 3D
object,
Fig. 1, One original photograph of the wooden house (source: Li,
[1999, p. 123])
the geometry in the image on a photograph of the object is
generally not conforming with that on the corresponding line map
of the same object. The traditional line map is mostly a
Transverse Mercator Projection of the earth surface area onto
the map plane, and for an area small enough, it is equivalent
with the direct orthogonal projection onto the map plane. For the
case of the regular engineering drawing of any architectural or
industrial structure, it is always an orthogonal projection from the
object onto the drawing plane. Consequently, for the mapping of
a properly small area or the engineering drawing of a structure,
ortho photo is the only type of photograph containing images
conforming with the corresponding map or drawing in geometry.
The original photograph of certain land area can be
photogrammetrically processed by rectification to become an
ortho photo. Thus when a digital air-photo is introduced as a
layer to be superimposed with the corresponding line map in GIS,
the original photograph needs to be first rectified to become an
ortho-photo, in order to ensure the match of the photograph with
the line map. Although air-photo is generally not an issue in
close range photogrammetry but an issue in conventional aero
photogrammetry, the issue of ortho photo is a commonly
encountered question in GIS. For the applications of DCRP in
GIS, one commonly useful case is that certain planes on the
object imaged in the digital photograph are selected respectively
as the ortho photo planes of different corresponding parts of the
object image. This can be easily accomplished by PM Pro.
Figure 2 is the ortho photo of a part of one of the four vertical
wall planes (the second vertical wall plane from the right),
produced from the photograph in Figure 1 by PM Pro.
Fig 2. Ortho photo of one wall plane of the wooden house
(source: Deng, [1999, p.83j)
It can be easily seen that the oblique angle of projection for this
wall plane in Fig. 1 is “rectified’ in the ortho photo. Once the
scale of the ortho photo is given, correct distances can be
directly and conveniently measured on any part of the ortho
photo, because the geometry in an ortho photo is equivalent to
that in the corresponding engineering drawing or line map
formed by orthogonal projection. On the other hand however, it
is impossible to guarantee the correct geometrical measurement
on the original photograph as seen in Fig. 1 without rectification
2. Direct Measurement on digital photograph: For the
DCRP processing (which may include analytical processing and
image processing) of the digital photographs in PM Pro, a set of
points on the object is marked on the digital photographs by
simple mouse clickings to sub-pixel precision. Once the
processing is finished, the 3D coordinates of these points in a
pre-chosen object space coordinate system are determined, and
the geometrical measurement can then be made directly on the
digital photographs according to the marks of these points. The
measurements can be made by the convenient mouse clickings
on the point marks, to immediately get the 3D coordinates of a
point, the distance between two points, or the area of a selected
plane. Figure 3 illustrates the direct measurement of the window
width on the digital photograph in Figure 1. By selecting distance
measure mode and clicking on the two end points or the joint
line between the two points on the lower side of the window, the
length (window width) of the line between the two points is
