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PHOTOGRAMMETRIC
WORKFLOWS
Traditional, Digital and the Future
Terrain Generation: Digital orthophotos
are one of the primary end-products in the photogrammetric workflow. Accurate terrain models are an essential ingredient in the generation of digital orthophotos.Terrain models can take the form of Triangulated
Irregular Network (TINs) or grids.Once AT is complete, terrain generation can typically be run as an automatic
process in most photogrammetric packages.Automatic terrain generation algorithms typically match “terrain points” on
two or more images (more images increase the reliability of the point). Seed data such as manually extracted vector
files, control points, or other data can often be input to help guide the correlation process. There are usually filtering options to remove blunders, also referred to as “spikes” or “wells” in the output terrain
model. Filtering can also be used to assist in the removal of surface features such as buildings and trees. This can be of great assistance if the desired output is a “bare-earth” terrain model. It is important
to note that terrain may also be acquired via manual compilation (in stereo), LIDAR,or publicly available datasets (e.g. SRTM).
Terrain Editing: Digital Terrain Models
(DTMs) that have been generated by
autocorrelation procedures typically
require some “cleanup” activities to model
the terrain to the required level of accuracy.
Most photogrammetric packages
include some capability of editing terrain
in stereo. It is important for operators to
see the terrain graphics rendered over
imagery in stereo so they can determine if
automatically generated terrain posts are
indeed “on the surface.” That is, that the
DTM is an accurate (or accurate enough
for the specific project) representation of
the terrain. Terrain can usually be rendered
using a mesh, contours, points, and
breaklines. The operator usually has control
over which rendering method is used
(it could be a combination) as well as various
graphic details such as contour spacing,
color, and line thickness. Terrain editing
applications usually provide a number
of tools for editing TIN and grid terrain
models. In addition to individual post editing
(e.g. add, delete, move for TIN posts,
adjust Z for grid cells), area tools can also
be used for a number of operations.
These may include smoothing, surface fitting
operations, and spike and well
removal tools. Geomorphic tools can be
used for editing linear features such as a
row of trees or hedges. After a terrain edit
has been performed, the system will
update the display in the viewer so the
operator can assess the accuracy and
validity of the edit. Once the editing
process is complete, the user may have to
convert it into a customer-specified output
format (e.g. one TIN format to another,
or TIN to grid). DTMs are increasingly a
customer deliverable (and product in
themselves), as they have many uses and
Fig. 2 Traditional Photogrammetric Workflows
Feature Extraction: Planimetric feature
extraction is usually an optional step in
the workflow, depending on the project
specifications. Automatic 3D feature
extraction algorithms are under development,
but manual stereo extraction is still
the predominant method. Feature extraction
tools in digital photogrammetry packages
typically allow users to collect, edit
and attribute point, line, and polygonal
features. Features can be products in
themselves, feeding into a 3D GIS. Alternative
building futures may be used again
in the photogrammetric processing chain
in the production of “true orthos.” This
process takes surface features into
account to produce imagery with minimized
building lean which can be particularly
beneficial in urban environments.
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