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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
and is with respect to terrain that is moderately sloped, bare
(DSM) and unobstructed. Details may be found at
www.intermaptechnologies.com. These specifications apply to
X-Band products only.
3. NATIONAL MAPPING PROGRAMS: NextMap
Because of the acquisition capacity demonstrated by STAR-3/
and improvements in the processing chain, it has become
feasible to perform national mapping programs at the
specifications listed (Table 2) over reasonable periods of time.
Intermap’s name for this type of program is Nextmap. The first
such program done with a 1| meter or better vertical
specification was NextMap Britain.
3.1 NextMap Britain
The NextMap Britain program was performed in two phases.
Phase | saw the acquisition, processing, delivery, validation,
acceptance and release of the data performed over an 18 month
period starting in December 2002 for an area that included
England, Wales and the southern portion of Scotland (Figure 3).
In Phase II, the Northern part of Scotland was acquired with
delivery occurring now and release scheduled for July, 2004.
The Phase 1 requirement, was for Type II DSM and DTM
delivery over about 150,000kmsq with Type | specification for
about 50,000kmsq of the low-lying, flood-prone SE part of
England.
Figure 3. Part of NextMap Britain, this shaded relief image of
the resulting DTM includes England and Wales (from Duncan,
et. al., 2004). The data for this example were acquired during
about three months of night-time flying followed by one year of
processing and editing.
843
Deliveries of DSM and DTM and ORI for the whole of
Nextmap Britain include 2800 tiles (10km x 10km) referenced
to the British Ordinance Survey grid.
One of the key operational factors in this rapid acquisition was
the use of long flight lines — 200km over much of the area. In
order to remove systematic errors, orthogonal tie lines were
flown every 50 km and were tied down by trihedral corner
reflectors that had been surveyed in with DGPS. GPS
baselines for the airborne acquisition were maintained within 75
km for the Type II specification and less for the Type !
acquisition. Type Il acquisition occurred at 28,000' — 30,000’
flying altitudes, while Type I was flown at 20,000” — 22,000? to
improve signal-to-noise performance as noted above in section
2.
3.1.1 Validation: Several forms of validation have been
reported on in the course of this program, both external
(Dowman, (2004) and Duncan, et. al. (2004)) and internal
(Mercer, et. al., 2003). Results are consistent among these
studies although they vary somewhat in their approach.
Dowman's approach was to examine a sub-area in great detail,
inter-comparing lidar, GPS, photogrammetric and IFSAR
results in different terrain and terrain cover situations.
Comparing the IFSAR results to these forms of truth, the DSM
and DTMs varied from (approximately) 0.5 m to 1.1 m RMSE
in various conditions.
The approach of Duncan, et. al. was to take advantage of the
relatively wide-spread lidar (Optech ALTM 2033) and GPS
ground sample coverage that had been acquired by the
Environmental Agency. The lidar DTMs themselves,
originating with ground samples at 0.25 — 2 meter sampling
density, had been validated in the vertical at better than 10 cm
RMSE according to some 627 GPS test sites distributed around
the country. Of these 322 GPS test sites were used for the
IFSAR comparison, each with at least 100 points contributing to
the derived RMSE. Similarly, some 595 lidar test sites were
used for the IFSAR comparison, with several thousand points
per site contributing to the statistics. All test points were
collected at least 6 meters from the nearest building or other
obstruction, to prevent biasing the statistics. The DTM results
reported by Duncan, et. al. (2004) are summarised in Table 3.
DTM (IFSAR - GPS) (IFSAR - LIDAR)
Statistics| # Sites | <RMSE> || # Sites | <RMSE>
Type | 130 0.51 234 0.64 [meters
Type II 192 0.75 361 0.92 [meters
Table 3. Summary of observed site statistics for IFSAR DTM
differences with respect to GPS and Lidar (Duncan, et.al., 2004).
Note that <RMSE> refers to the mean RMSE taken over all of
the test sites. Type I and lI refer to the areas (50,000kmsq and
150,000kmsq in size, respectively) where the different
acquisition specifications established by Intermap (Table 2)
were implemented. See details in text.
It should be noted that further breakdown of the test sites into
those in urban and non-urban areas showed the errors in the
former to be larger (approximately 3096) than in the non-urban
areas. Table 3 comprises both urban and non-urban sites.
In the tests performed by the author, (Mercer, ét. al. (2003), a
smaller (33 lidar tiles, each 2km x 2km ) but well distributed
sub-set of the same lidar DSM data provided by the
