International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
Be
Semi-Permanent’ Ground Marker
Survey using DGPS
‘Accuracy verification’
^
A. ABEMGY
Figure 6. Specification of the UK Highways Agency control
panels with location along the highway indicated.
8. COST AND ECONOMY
There is a clear separation between the costs and the
applications of airborne LiDAR and IfSAR. LiDAR is
generally high cost and high accuracy, and suited to covering
small areas. IfSAR is less accuracy but can cover large areas
very economically. Mercer (personal communication) quotes
costs of about $5 per km? for the IfSAR data of Nextmap
Britain and about $500 per km? for Airborne LiDAR. Wulder
(2003) quotes Canadian $1900 per km? for LiDAR with 30cm
posting. Some of the shortcomings of IfSAR can be overcome
at additional cost. For example problems with occlusions can
be solved if an area is flown from two look directions.
There are some important points to consider when looking at the
accuracy of products:
* The product must be fit for purpose with suitable
accuracy and point density
* The data must fit to other data - very important with
web delivery to non specialist users
* Accuracy must be specified
9. STANDARDS
As LiDAR becomes more widely used, it becomes more
essential that data can be easily read by standard mapping
systems, and passed between users. This implies a need for
standards and interoperability.
In the USA a standard has been established for the formatting
of LiDAR data (http://www.lasformat.org/). The LAS format
has been accepted by the American Society for Photogrammetry
and Remote Sensing (ASPRS) and is being widely used This
has also been incorporated into the Highways Agency
Specification for LiDAR in the UK. In the USA, FEMA
(2000) has a standard specification for LIDAR and ASPRS are
developing a handbook of operational LIDAR Mapping. ISO
19130 TC 211 "Geographic information / geomatics" project
team of "Sensor and data models for imagery and gridded data"
has published a Committee Draft of an ISO standard for
photogrammetry and remote sensing but this does not include
sensor models for LIDAR and SAR.
It is important that these initiatives are carried forward; this is
an opportunity to define standards for a new sensor and new
products at an early stage in their development.
96
10. SPACEBORNE IfSAR
Spaceborne IfSAR is more established as a source of DEMs that
is airborne. The ESA ERS Tandem mission has acquired very
wide coverage of interferometric SAR pairs and this is much
used for the generation of regional DEMs. For example the
Radarmap of Germany produced by DLR (Kosmann et al, 1994)
and the Landmap project in UK (Morley et al, 2000). The
Shuttle Radar Topography Mission (SRTM) has also produced
DEMs and orthoimages between 60° North and 56° South. In
addition RadarSat, JERS, and ENVISAT all produce
interferometric data and in the future RadarSat 2 and ALOS
PALSAR will join the ranks of IfSAR data generators. IfSAR
has also had an important application in differential mode for
monitoring tectonic movement and subsidence.
With the exception of SRTM, satellite IfSAR uses repeat pass
data, and this can suffer from the problem of poor coherence
and atmospheric effects, which degrade the data and can cause
gaps in the DEM. SRTM also suffers from problems,
particularly layover in mountainous areas, and the SRTM
dataset does contain some gaps.
The accuracy of IfSAR DEMs from spaceborne platforms varies
significantly, depending on the coherence, itself dependent on
the interval between acquisition of the two images and stability
of the weather and atmosphere, and the terrain.
11. DATA FUSION
Data fusion exploits the synergy of two or more data sets to
create a new data set which is greater that the sum of the parts.
The ISPRS Journal of Photogrammetry and Remote Sensing
(Vol 58(1-2), 2003), published a theme issue on multi-source
data fusion for urban areas which clearly demonstrates the range
and importance of data fusion. Data fusion can be used for
many applications. Some of the established ones are:
* Assisting phase unwrapping
* Eliminating errors and blunders
e — Atmospheric correction
* Providing orientation in areas where there is no
control
e Terrestrial images to LIDAR
e Feature extraction, such as buildings and roads
* Other aspects of feature extraction and environmental
analysis (see ISPRS Journal 58(1-2)).
Some examples of how DEM data from LiDAR or IfSAR can
be combined with other imagery or map data for feature
extraction are given in section 13.5. Honikel (2002) shows how
ERS If SAR and SPOT DEMs can be fused and develops a
theory for this; Csanyi and Toth (2003) also discuss the
theoretical aspects of merging IfSAR and LiDAR data.
SRTM provides another interesting case study. Because SRTM
provides a near global data set which is geocoded with accuracy
which is better than any other comparable global DEM, it can
be used to give initial orientation for higher accuracy data and
be used to assist with phase unwrapping and atmospheric
correction.
12. USER CONSIDERATIONS
LiDAR and IfSAR are relatively new products and it is
therefore necessary to overcome the reluctance of users to make
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