The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
The resulting 10 cm standard deviation (random error), Table 5, 
is about double the one obtained using static reference point 
measurements. 
Standard 
deviation 
Reference points 
Search circle radius 
0.10 
76 
R<0.5m 
0.11 
251 
R<1.0m 
Table 5. Height accuracies of Leica ALS50-II laser points on 
the asphalt road, (laser points-reference points) 
The accuracy of the mobile measurements was estimated by 
measuring 51 static reference points with VRS-RTK GPS. 
Mobile derived XY co-ordinates were searched within 10 cm 
distance on the road. Results are in Table 6. We can say that the 
mobile reference point measurement system gives the accuracy 
that is good enough for the quality and gross error checking of 
ALS measurements. 
Mean difference in Z 
-0.004 m 
Standard deviation 
0.036 m 
Max positive difference 
0.118m 
Max negative difference 
-0.063 m 
Table 6. Accuracy of mobile VRS-RTK. Mobile - static 
reference point measurement. 
4. CONCLUSIONS 
In this article we have studied different aspects of the quality 
checking of the laser based nationwide digital elevation model 
using the Salo test data. 
The data of two ALS systems, Optech ALTM 3100 and Leica 
ALS50-II, have been analyzed and found to be of high quality. 
The flight altitude of 4750 m gives such a sparse pulse density 
that a 2 by 2 m 2 grid model is unrealistic to create even though 
the obtainable elevation accuracy is high, partly thanks to the 
relatively flat test site. Significantly higher errors are expected 
to occur in more steep terrain and when obtained models are 
compared to the reference; in this comparison we always had 
laser and reference point close to each other (maximally, either 
0.5 m or 1 m away from each other). 
The quality obtained in all various surface types was better than 
the specified 30 cm. 
The obtained planimetric accuracies of the scanners from the 
altitude of 2000 m were roughly within the specifications of the 
scanner manufacturers. 
Black asphalt does not always reflect laser beam well enough 
for accurate measurements from 2200 m flying height. This 
causes the lack of laser points on the asphalt roads. 
Results show that a mobile VRS RTK measurement system is a 
cost effective way to collect large amount of reference points 
that are cost-effective and accurate enough for checking the 
quality of ALS, especially for quality control of strip adjustment 
and quality checking of homogeneity of the data over large 
areas. Levelling and static reference point measurements are 
though needed in specific areas where more precise reference 
points are needed. 
The test project showed that the airborne laser scanning 
parameters for fulfilling the nation-wide elevation model 
accuracy specifications could be as follows: Scan angle of ±20 
degrees, flying height of about 2000 m, point density at least 0.5 
points/m 2 , side lap 20 % minimum. 
After the strip adjustment and the elimination of systematic 
errors in the laser data of the national laser scanning vertical 
accuracy of the unambiguous planar areas will be better than 15 
cm (RMSE). Without strip adjustment, there were even errors 
up to more than 1 m, thus, the strip adjustment is a needed 
procedure even at nation-wide data collection. The required 30 
cm RMSE of the new 2 by 2 m 2 grid model for unambiguous 
planar surfaces will be met with 0.5 pulses per m 2 density. 
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ACKNOWLEDGEMENTS 
The co-operation with Juha Vilhomaa, Risto lives and Olli 
Sirkia from the National Land Survey of Finland is highly 
acknowledged.