fig. 3 Contour plot of groundwater level above mean sea level. 
ad 1) 
In dune areas land form and land cover are 
the most important features for analysis 
and classification (Van der Meulen en Jun- 
gerius, 1989). To describe the landscape 
succession in the past 50 years the vegeta 
tion structure was used. Vegetation struc 
ture is a main part of the landscape and it 
can easily be distinguished in aerial pho 
tographs. Land form was only used as a help 
by delineating homogeneous units. 
ad 2) 
Vegetation structure can be divided in two 
aspects: vertical stucture (profile) and 
horizontal structure (pattern). The cove 
rage of the tree, shrub, herb and moss 
layers were used to select different photo 
units. These photo units are comparable 
with vegetation formations (Tans- 
ley,1946).When the treelayer is absent and 
the shrub vegetation covers over 75 percent 
, that photo unit was classsified as dense 
shrub. When vegetation is absent or covers 
less then 25 percent of the airphoto unit 
was classified as sand (with small pionier 
vegetations). In the very simplified deter 
mination key (fig. 4) the main structure 
types are shown: woodland, dense and open 
shrub, moss and herb vegetation and absence 
of vegetation. A total of 9 formations were 
recognized (tab 2). 
Photo interpretation,however, is not to 
tally objective. Therefore it was necessary 
to analyse and classify in a systematic 
way, by means of a dichotomous determina 
tion key that could be used for the fal 
se-colour diapositives as well as for the 
black- and white photographs . It is inevi 
table that,when using the same determina 
tion key for both types of photographs , 
information is lost when interpreting the 
false-colours. Moss- and herb vegetations 
for example can easily be distinguished at 
the false colour photos, but at the black- 
and white- photographs you can't tell which 
is which. So this difference (=information) 
can't be used in a determination key for 
both types of photos.To do more detained 
studies hierarchical subunits can be for 
med. 
SIMPLIFIED DETERMINATION KEY 
1. a) coverage treelayer > 25% WOODLAND 
b) " ” < 25% go to 2 
2. a) coverage shrublayer >50% and 
coverage sand/moss/herb layer <25%.... DENSE 
SHRUB 
b) coverage shrublayer >25% OPEN 
SHRUB 
c) else 9° to 3 
3. a) coverage vegetation < 25% SAND 
b) else MOSS/ 
GRASS 
fig. 4 Simplified determination key 
ad 3) 
In 1985 field studies were carried out to 
check the photo interpretation map of the 
1985 photos, and to get an impression of 
the vegetation types representing the dif 
ferent structures. 
The photo interpretation maps were adjusted 
to 1:5000 scale, regarding the total area 
and surrounding roads. 
4 RESULTS 
4.1 Field check 
The results of the field studies were rat 
her satisfactory. Ninety percent of the 
photo map units were given the same struc 
ture type at the field check. Of the remai 
ning 10 percent particularly the coverage 
its true value. Units with mixed low and 
tall shrub could much easier be classified 
and delineated with the aerial photographs. 
4.2 A general desription of the changes 
1938-1985 
A process of growth in the vegetation 
structure is manifest. In 1938 (fig. 5) it 
is obvious that open moss/herb/grass vege 
tations were dominating. Notable are the 
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