900 
The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B5. Beijing 2008 
Figure 6.Computed cross sections and the reconstructed model of a hand 
6. DERIVING THE INTERNATIONAL ROUGHNESS 
INDEX (IRI) 
The IRI measurement has m/km (sometimes mm/m) or in/mi 
units. The determination of IRI can be easily done from the 
intersected surface points; only the longitudinal profile has to 
be used. The most relevant place of road roughness is exactly 
under the tires, therefore the four (2 left and 2 right) markers in 
line with the tires have been selected for IRI calculations. The 
IRI values are calculated for all four points, samples taken in 
every 80 cm (at a speed of 4 m/s [15 km/h]) along the road, then 
their average are ordered to the analyzed road segment. 
A further software module has been developed, which produces 
IRI files (reports) and IRI maps. In Figure 7 the effect of the 
recently finished road reconstruction on the bridge and the lack 
of it on the east (Pest) side can be clearly seen. 
Figure 9 shows the results of a 56 km long surveying campaign 
produced by the latest version of the measurement system in 
April 2008. Figure 10 is an enlargement to visualize some 
details of the survey. 
There were registrations in three different days, where the 
measurement repeatability and accuracy can be analyzed. The 
system classifies the road segments into six categories 
depending from their IRI values: Cat. 1 is the best and Cat 5 the 
worst. The special sixth category is used for non-measured 
segment, where the GPS-positioning was failed (e.g. because of 
urban canyon effect, under bridges) or the IRI-measurement had 
any other problem. The following table shows the quality 
distribution of the surveyed roads for the five longest sections: 
The IRI calculation is done at the highest resolution, but of 
course the values can be aggregated arbitrary. 
An additional experiment was also performed: the potential of 
vertical acceleration directly in IRI calculation i.e. how can IRI 
be derived from the single vertical acceleration measurements. 
To test this hypothesis, regular gridded vertical acceleration 
values were interpolated, then they were integrated into vertical 
movements, and at the end the IRI-calculations were performed 
as (Sayers & Karamihas 1998) refers. The IRI-values are 
calculated for 2 m long segments. Since only a single IMU was 
applied in the vehicle, no smoothing (e.g. averaging) was 
needed (compared to the previously mentioned “optical” IRI 
method). 
Buda Danube Pest 
Figure 7. IRI map derived from laser markers below the tires, 
(light is for good, dark for bad status.) 
SI 
S2 
S3 
S4 
S5 
Cat. 1 
2606 
1340 
1475 
518 
979 
Cat. 2 
4456 
1797 
1968 
978 
842 
Cat. 3 
2408 
1269 
707 
753 
402 
Cat. 4 
1228 
737 
362 
466 
223 
Cat. 5 
1785 
1143 
401 
773 
342 
Cat. 6 
2266 
1615 
336 
820 
946 
Length 
14752 
7904 
5251 
4312 
3737 
Mean 
IRI 
9.32 
10.00 
7.70 
11.54 
8.28 
IRI SD 
7.28 
7.51 
6.72 
8.52 
7.19 
Table 1. Basic statistics for the five longest sections measured in 
the last campaign. The categories and length are given in m, IRI 
in m/km 
Figure 8. IRI-values calculated directly from vertical 
acceleration measurements on different pavement types