ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision“, Graz, 2002 
ASSESSING HEIGHT PRECISION OF LASER ALTIMETRY DEMS 
Marc Crombaghs, Sander Oude Elberink, Regine Brügelmann, Erik de Min 
Ministry of Transport, Public Works and Water Management, Survey Department, 
P.O. Box 5023, 2600 GA Delft, The Netherlands 
{m.j.e.crombaghs, s.j.oude-elberink, r.brugelmann, e.j.dmin}@mdi.rws.minvenw.nl 
Commission III, WG III/3 
KEY WORDS: Laser altimetry, DEM/DTM, quality, height precision, strip adjustment 
ABSTRACT: 
The new Dutch national digital elevation model (DEM), acquired with laser altimetry, is almost complete. The Dutch Survey 
Department fulfils an intermediary function between laser data suppliers and final users of the DEM. One of the most important tasks 
of the Survey Department is to guarantee the quality of the delivered laser DEMs to the users. For this purpose, a new height error 
description scheme is developed, allowing to quantify error effects at different scales. Such a new error description scheme was 
necessary because the former quality description of DEMs was insufficient. The error behaviour of laser altimetry data, acquired by a 
complex system of different sensors, cannot be expressed by solely two parameters: a bias and a standard deviation. The former and 
the new error description scheme will be addressed together with methods to quantify the differently scaled error components. 
Among them are cross correlation techniques, empirical covariance function analysis from geostatistics and 1d strip adjustment. The 
contractual demands for maximal allowed error amplitudes, derived from real data, will be presented. The benefit of the error 
description scheme for DEM users will be illustrated by propagating the error components to the height precision of derived 
products. Summarizing, it can be stated that this paper deals with assessing the height precision of laser altimetry DEMs and 
quantifying the effects of the different error components on the measured heights. This paper does however not aspire to give 
methods for eliminating or minimizing these errors. 
1. INTRODUCTION With increasing insight into the occurring errors in laser 
altimetry data, we realized that strip adjustment could 
Since 1996, the new Dutch national digital elevation model, strongly improve the laser DEM. Furthermore, strip 
the AHN (Actual Height model of the Netherlands), is being adjustment provided the possibility to get a thorough and 
acquired using laser altimetry. The new DEM is expected to comprehensive insight into the quality of the delivered laser 
be completed in 2002. This project was initiated to meet the altimetry data. A 1D strip adjustment procedure was 
demand for detailed and actual information about elevation developed at the Survey Department correcting for strip tilts 
from water boards, provinces and "Rijkswaterstaat" (Ministry and height offsets [Crombaghs et al. 2000]. This requires, 
of Transport, Public Works and Water Management). The however, the delivery of the laser data in individual strips. 
former elevation model, over 40 years old, has become Also further correction tools were created in order to 
obsolete. Moreover, it has a much lower point density than eliminate possible occurring cross strip parabolic effects and 
the AHN: 1 point per 10.000 m° in opposition to 1 point per to handle strip torsions or periodic effects. 
16 m° in average. 
This way, the Survey Department began processing and 
The Dutch Survey Department (Meetkundige Dienst Rijks- correcting the laser data itself on a large scale. This appeared 
waterstaat) is co-ordinating the acquisition of the AHN. This to be a time consuming task. Furthermore, this did not 
task comprises the contracting of companies for the laser correspond with our ambition to be a controlling and quality 
altimetry flights as well as for the processing of the original guaranteeing authority. Thus, the Survey Department 
data to get X, Y, Z terrain co-ordinates and for the filtering. encouraged the laser scanning companies to perform strip 
In addition, the quality of the delivered laser data is checked adjustment themselves by, among other things, placing the 
at the Survey Department and standard products, e.g. 5m x strip adjustment software at their disposal. In addition, some 
5m grids, are derived and distributed to the participants such companies also did further efforts to improve the accuracy of 
as the local water boards and provinces. The Survey laser data, e.g. by more thorough calibration procedures and 
Department also performs research to improve the AHN additional control mechanisms. 
quality and to investigate further applications of laser 
altimetry ([Huising and Gomes-Pereira 1998], [Crombaghs et Now that the first version of the AHN is almost complete, the 
al. 2000], [Brügelmann 2000]). Survey Department wants to retire from the data correcting 
task (which is expected to be done by the laser scanning 
In the very beginning of AHN acquisition, the laser data was companies) and to return to the originally aspired data 
not delivered in individual strips. Controlling the height certificating task. For this purpose, a new height error 
precision consisted of checking the height differences at description scheme is developed for DEMs acquired by laser 
some horizontal and flat ground control regions (‘control altimetry, allowing to quantify error effects at different scales. 
points’) spread over the laser block. This way only a fairly Such a new error description scheme was necessary because 
limited sample test was possible for small parts of the whole the former quality description of DEMs was insufficient for 
DEM. describing the whole error budget of a single laser point or of 
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