Automated Road Extraction and Updating Using
the Atomi System - Performance Comparison
Between Aerial Film, ADS40, IKONOS and
Quickbird Orthoimagery
2. BRIEF DESCRIPTION OF PROJECT ATOMI
2.1 Aims of ATOMI
The aim of ATOMI is to update roads digitised from 1:25,000 scale maps by fitting them to the
real landscape, improve their planimetric accuracy to 1 m and derive road centerline heights
with an accuracy of 1 to 2 m. The topology and the attributes of the existing datasets should be
maintained. This update should be achieved by using the image analysis techniques developed at
the Institute of Geodesy and Photogrammetry (IGP), ETH Zurich. The whole procedure should
be implemented as a standalone software package, should be operational, fast, and most
importantly reliable. We do not aim at full automation (ca. 80% completeness is a plausible
target), but the “correct” results should be really correct to avoid checking manually the whole
dataset. More details of ATOMI can be found in Eidenbenz et al. (2000). The standard input
data includes 1:16,000 scale colour imagery, with 30 cm focal length, and 60%/20%
forward/side overlap, scanned with 14 microns at a Zeiss SCAI, a nationwide DTM (DHM25)
with 25 m grid spacing and accuracy of 1-3/5-8 m in lowlands/Alps, the vectorised map data
(VEC25) of 1:25,000 scale. The VEC25 data have a RMS error of ca. 5-7.5 m and a maximum
error of ca. 12.5 m, including generalisation effects. They are topologically correct, but due to
their partly automated extraction from maps, some errors exist.
2.2 The Road Reconstruction System
The developed system makes full use of available information about the scene and contains a set
of image analysis tools. The management of different information and the selection of image
analysis tools are controlled by a knowledge-based system. We refer to Zhang (2003a) for more
details. The system can extract roads with a minimum width of ca. 3 pixels. It focuses on
extraction of roads in open rural areas. The system has been modified to work also with
orthoimages, whereby the 3D information is extracted by overlaying the 2D information on the
DSM or DTM. Our system includes tools for external evaluation of the extracted results, by
comparing the extracted results with precise reference data. The quality measures aim at
assessing completeness and correctness as well as geometric accuracy. Completeness measures
the percentage of the reference data that lies within the buffer of the extracted roads, while
correctness is the percentage of the extracted roads within the buffer of the reference data. The
buffer distance is defined using the required accuracy of the project ATOMI, i.e. 1 m. The
geometric accuracy is assessed by the mean and RMS of the distances between the extracted
roads and the reference data. The detailed description for the computation of the external
evaluation measures is presented in Zhang (2003a). The developed system has been
implemented as a stand-alone package initially on SGI platforms for stereo and orthoimages and
has been ported to Windows XP only for orthoimage processing, with the same user interface.
The system imports imagery, the existing road database and height data, and outputs the
extracted road network as well as the computed road attributes including length and width in 3D
Arc/Info Shapefile format that is readily imported into existing GIS software. The Windows XP
version for orthoimages is termed ATOMIRO (with R standing for roads and O for
orthoimages). All current and further improvements of the system and the tests reported here
refer to ATOMIRO, while the SGI versions have been frozen.
3. TEST SITES AND DATA DESCRIPTION
Results from two test sites in Switzerland will be presented here, one in Thun and the other one
close to the city of Geneva. The selection is mainly based on the consideration (1) the test sites
should cover as many types of typical landcover in Switzerland as possible, and (2) the
availability of images from multiple sensors. Both sites are in open rural areas but with different
landcover. All road types in Switzerland can be found in the areas. The description of the test
sites and the available imagery are listed in Table 1. Fig. 1 shows aerial images of the two test
sites. Much larger and different regions have been used for tests by swisstopo with a total road
length of about 9,000 km.
Table 1. Test site description and image specifications.
Figure 1. Overview of test sites: (a) Thun, (b) Geneva.
In Thun, the orthoimages were produced by swisstopo from aerial images of scale 1:16,000
using the DHM25. The 50-cm orthoimage is part of the nationwide dataset Swissimage with a
planimetric accuracy of about 1 m. The images for 20 cm and 60 cm were taken in spring 2003
and for 50 cm in summer 1998. An orthoimage created from ADS40 summer images using the
DHM25 is also available. Due to weaknesses in the control point distribution and the bundle
adjustment of the ADS40 images, a discrepancy between the ADS40 orthoimage and the 20-cm
orthoimage has been observed. A non-exhaustive comparison with manually selected feature
points shows that the discrepancy varies between 0 and 80 cm. However, smaller differences
also exist between the 1998 and 2003 aerial film orthoimages, caused by errors in the sensor
orientation. Thus, the real accuracy of road extraction in image space is higher than the accuracy
values derived from comparison between datasets (incl. the reference data), which have varying
orientation errors. The Geneva test site (Fig. 1b) is near the city of Geneva, containing several
larger villages, forest and a river. Another difference to the Thun site is that the scene contains
grasslands and large fields of bare soil. In addition, many road-like lines are observed in the
fields. The aerial orthoimage came from the Swissimage dataset. IKONOS and Quickbird
images were also acquired in May 2001 and July 2003 respectively. The pansharpened (PSM)
orthoimages of IKONOS and Quickbird were produced by software developed at IGP using a 2-
m grid laser DTM with 0.5-m accuracy and had a planimetric accuracy of 0.5-0.8 m. The
reference data for the Thun and Geneva test sites were measured manually, by swisstopo in 20-
cm pixel size aerial orthoimages and by ETH Zurich in the Swissimage orthoimages,
respectively. The tests were performed on a DELL PC with Pentium 4, 1.8GHz CPU and 1GB
RAM running Windows XP.
