FAQ This section contains answers to some questions often coming up when working with the PHOTOMOD
system.
If you cannot find an answer to your question here, our
Technical Support Service is always ready to help you in solving the problems.
Computer setup to work with PHOTOMOD
Q: What are the minimum requirements and recommended configuration
for the PHOTOMOD workstation system?
A: See Hardware Requirements.
Q: In the "Hardware Requirements" section it is stated that "...some additional
setup and tuning of OS is necessary for efficient working under Windows XP". What does it mean?
A: See Recommendations on optimizing Windows XP for working with PHOTOMOD.
Q: Are there any compatibility problems working with PHOTOMOD under Windows XP
Service Pack 2?
A: The Data Execution Prevention technology, implemented in Windows XP SP2, may cause problems when running
some PHOTOMOD modules on PCs with modern CPUs. See PHOTOMOD and Windows XP SP2 for details.
PHOTOMOD AT
Q: Threshold value of vertical parallax residuals
A: Usually should not exceed image pixel size. If vertical parallax residuals exceed
image pixel size significantly you should check the following:
- Conformity of tie points position on the left and right stereopair images
- Interior orientation – marks should be measured in appropriate order
- Correctness of camera parameters input. Typical mistake for instance
is distortion corrections input in micrometers instead of millimeters
Q: Minimal number of ground control points on airborne images block
A: Minimal ground control points number on image block of any size is theoretically 3
(in this case the block of standard configuration would be adjusted).
In practice it is recommended to have GCP for each five-seven strip bases
Q: Minimal number of ground control points on the single airborne image
A: 3
Q: Is it possible to make aerial triangulation using only projection centers?
A: Yes. Nevertheless, it is recommended to have at least 2-3 ground control points.
Also it is undesirable to adjust one strip by projection centers which are located
on the “straight line”.
Q: Minimal number of ground control points on the stereopair or single scanner
spaceborne image
A: Recommended points number – 10. Algorithms using precise sensor model (SPOT,
ASTER) or RPC coefficients (IKONOS, QuickBird) allow to decrease the number
of ground control points needed.
PHOTOMOD Solver
Q: What adjustment method should be used
A: Independent strips – to detect possible blunders
Independent stereopairs – for the final adjustment
Q: Adjustment blunders causes and their removal
A: The common recommendation is to control tie errors on the stage of data collection
in PHOTOMOD AT module using vertical parallax residuals and models tying. The typical sources
of blunders are:
- Tie points measuring errors. Use “Free model” adjustment to find tie blunders.
If tie blunders occur you could exclude tie points from adjustment or re-measure them in
PHOTOMOD AT. If there no tie blunders the second most likely reason – errors in ground
control points recognizing on the images or incorrect input of their coordinates
- Left and right coordinate systems are muddled up
In case of huge block you should adjust it by parts – for instance by
strips or strip pairs to detect block parts with maximal errors
In some cases you can detect blunders using preliminary adjustment by
independent strips method
Q: What level of adjustment accuracy could be achieved?
A: RMS on GCP values are generally comparable with the initial image pixel size
on the terrain. Naturally, the initial coordinates of GCP should be detected
more precisely. You should also pay attention that scanning resolution could
vary within reasonable limits, 15 – 30 micron could be considered as typical
resolution. In most cases resolution could be increased up to 7 micron that
is not so perceptible in adjustment accuracy due to the influence of photomaterial
quality.
Q: How the initial flight scale relates to the resulting map scale?
A: It depends mostly on initial GCP accuracy, required accuracy of resulting
map and scanning resolution. Standard and guaranteed ratio of input and output
scale is 5. In other words, if flight scale is 1:10,000 – map scale is 1:2,000.
Nevertheless this ratio could be improved up to 7-10. For instance, there is
aerial images block in 1:10,000 scale. Scanning resolution is 15 micrometers,
so pixel size on the terrain is 15 cm. Expected adjustment accuracy (RMS on
GCP) – is 15 cm. Required GCP accuracy for 1:1,000 map scale is 0.2 mm, so –
20 cm on terrain. Thus, if flight scale is 1:10,000 map scale will be 1:1,000.
PHOTOMOD DTM
Q: What TIN building method to use?
A: Adaptive with detalizing extended relief features using breaklines.
In case of large scale survey and also if there is dense building up or forest-covering
manual pickets collecting and breaklines vectorization and TIN building “by vectors” afterward
PHOTOMOD StereoDraw
Q: How to export attributes attached to vector objects?
A: Attributes attached to the vector objects (code table records and “additional”
unique attributes for each object) could be exported into DBF file using appropriate
export option. When exporting to MIF / MID format attributes will be saved to
MID file.
PHOTOMOD Mosaic
Q: How to create regions of interest and make cut lines?
A: The following cutlines rules are classic:
- Regions of interest borders (cut lines) should be drawn in the middle of
images overlapping areas, as a rule.
- Cut lines should not intersect buildings.
- Cut lines should not go lengthwise the borders of objects with different
tone.
- When intersecting linear objects and clear contours the cut line should be
drawn at right angles to these objects.
From brightness adjstment point of view (in case of color images mainly) the
following would be recommended:
- To make cut lines over the similar objects (which look “similarly“ on adjacent
images). At that “dark” objects are preferable rather than “light”.
- To draw the cutlines along borders of objects with the same brightness (for
example, fields, forests, water areas and so on), but with some indent to avoid
edge smearing effect when “seams feathering” option is on. The indent would
be single-double “Interpolation cell size” (see below). So if you are making
the cutline along the field, which is bordering the road you should draw it
on the distance from the road mentioned above and then intersect the road at
angles close to right.
- When using local brightness adjustment (see below) you should adjust images
in the strip and then make inter-strip brightness adjustment. Therefore insight
the strip or when transition from one ROI to the next their common border should
be continuous.
- More details about cutlines creation see in PHOTOMOD Mosaic module documentation.
Q: What is “Interpolation cell size”?
A: While building orthophotomap, irregular terrain model (TIN – Triangulated
Irregular Network) will be automatically re-calculated into regular one (DEM
– Digital Elevation Model), which will be used for orthorectification afterwards.
Maximal pixel number in DEM is the same as in output orthophoto (corresponding
to the parameter value – 1). However so much elements number is redundant in
most cases and moreover leads to significant slowing down of orthorectification
process. You can estimate DEM pixel size using element size of TIN created in
PHOTOMOD DTM module keeping in mind that its element should not be much less
than the smallest triangle. It is convinient to use breaklines (created in PHOTOMOD
DTM module) for the areas required higher accuracy (bridges, road junctions,
for instance) along with resampled DEM for the rest orthomosaic areas. It allows
to preserve orthorectification process speed without loss of accuracy.
“Interpolation
cell size” parameter value is 32 by default, it means that pixel size of calculated
DEM will be 32x32 times more than pixel size of output orthophoto. When using
external (imported) DEM, this parameter value is calculating automatically and
could be changed if resampling needed.
Q: Brightness adjustment of color images
A: It is recommended to use “local brightness adjustment” in combination with
“global adjustment by average brightness”. See above about cutlines creation.
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