Sub-task 4.2 Variation of Tree Heights within Stand
Digital Elevation Models (DEM's) for the entire Cwm Berwyn test area have
been derived and mosaiced using aerial photographs and remote sensing software
(Erdas OrthoMAX). The DEMs currently relate to two time periods; 1957 and 1995.
The DEMs describe the ground surface at the test site pre-planting (1957) and
post-planting (i.e. the tree canopy surface) (1995). These two surfaces alone
provide tree heights for the entire test area, permitting detailed studies of
tree height, and stand variability and the correlation of these values with
site factors such as distance from stand edge, soil type, exposure, topography
and a number of other variables. A visit by all three UK participant groups to
the Cwm Berwyn test area took place in June 1996, in order to obtain further
ground control points and ground truth measurements. These were used to improve
the model, to validate it, and will be used to establish which site variables
are best correlated with the DEM.
Sub-task 4.2
Digital elevation models have been successfully calculated using aerial
photography from the Forestry Commssion test site in Cwm Berwyn, Wales. The
purpose of deriving these models is to study the height variability in stands
with respect to other site variables with the aim of establishing a
relationship between the position of a tree in a stand and its characteristics.
This is a vital link between the individual tree scale at which many of the
damage models operate and the stand scale at which the results are most useful.
This is particularly important where a single measure is used as model input to
represent a whole stand, and yet it is unknown how much variability exists
within a stand, and therefore to what degree the results are representative of
the actual risk. Stand variability is also a crucial link between the models
operating at the tree level and the silvicultural strategies being employed
which will have different effects at different positions in a stand.
Sub-task 4.2
This Subtask involves the use of existing data, measurement of stand variability and modelling of the effect of variability. A great deal of data exist from the wind damage monitoring areas within the United Kingdom and the extensive sample plots maintained in Sweden. Statistical analysis is being used to define the variation in stand vulnerability from the detailed set of sample plots in Sweden. It is hoped to be able to identify the risk for a particular stand from the physical characteristics of a specified tree, for example the oldest tree within the stand. Suitable aerial photographs of monitoring sites within the United Kingdom, along with ground control points obtained with a GPS system, have been selected and sent to P05 for analysis of the stand variability. The model of forest canopy has been derived using digital photogrammetric techniques, using the aerial photographs and the GPS data as inputs and producing high resolution GIS data layers as outputs. One part of the validation of these data layers is via the use of new measurement techniques which are becoming available and allow rapid assessment of tree characteristics within a stand. The combination of GPS system and Criterion laser relascope for rapidly measuring tree diameter at breast height and position within a stand are being explored by P02 and the P04.
Sub-task 4.2
This Subtask involves the use of existing data, measurement of stand
variability and modelling of the effect of variability. A great deal of data
exist from the wind damage monitoring areas within the United Kingdom and the
extensive sample plots maintained in Sweden. Statistical analysis is being used
to define the variation in stand vulnerability from the detailed set of sample
plots in Sweden. It is hoped to be able to identify the risk for a particular
stand from the physical characteristics of a specified tree, for example the
oldest tree within the stand. Suitable aerial photographs of monitoring sites
within the United Kingdom, along with ground control points obtained with a GPS
system, have been selected and sent to P05 for analysis of the stand
variability. The model of forest canopy has been derived using digital
photogrammetric techniques, using the aerial photographs and the GPS data as
inputs and producing high resolution GIS data layers as outputs. One part of
the validation of these data layers is via the use of new measurement
techniques which are becoming available and allow rapid assessment of tree
characteristics within a stand. The combination of GPS system and Criterion
laser relascope for rapidly measuring tree diameter at breast height and
position within a stand are being explored by P02 and the P04.
Sub-task 4.2
A three dimensional model of the forest stand in the Cwm Berwyn forest,
Wales has been obtained from aerial photographs in 1957 and 1995. Experiments
have been carried out on the level of spatial resolution that may be derived,
while not exceeding the integrity of its quality based upon the source
documents and processing. Estimates have been made of the accuracy of the
derived X, Y and Z values of the canopy surface based on comparison with data
from 169 field observations using a GPS. The results show root mean square
estimates of X=0.79m, Y=0.86m and Z=0.85m. These figures suggest that the
height at a point in the forest canopy may be estimated to within 1m. Further,
some of the physical conditions in the vicinity of a pocket of wind damage may
be interpretable from visual observations of the ortho-corrected photography
for 1957 and 1995 and the 1995 elevation model (Subtask 4.2).
Sub-task 4.2
The results of the derivation of a model of the forest canopy from aerial
photographs shows good potential for analysis with respect to changes in
height, opennings in the canopy due to damage and variability in the surface of
the canopy. The geometric accuracy of the model derived from photography taken
in 1995 was higher in quality to that derived from the historical photography
of 1957. This was largely due to a lack of adequate information about the lens
distortions in the older camera. However, the central area of the older model
was sufficiently good to provide a model of the terrain pre-afforestation with
which to compare the, currently, forested landscape. From this model initial
studies are being carried out on the variability in the canopy height surface
and on tree height.
Sub-task 4.2
Examples of the preliminary datasets have been posted onto the WWW. The
example show the mosaiced DEM and the mosaiced orthophotographs. In addition
there are some perspective views of the site, derived by draping the
orthophotograph over the DEM and viewing the data from different locations, in
different directions.
The preliminary results have been presented to visiting groups to MLURI,
including senior representatives of the Forestry Commission, a visiting
delegation from Newfoundland and senior representatives of the Scottish Office
and Scottish Universities.
The following publications and scientific manuscripts have been published,
are in press, or in preparation within the first 24 months of Project:
Miller, D, Quine, C and Broadgate, M (1996), The application of digital
photogrammetry for monitoring forest stands. In: The Proceedings of
Application of Remote Sensing in European Forest Monitoring, Joint Research
Centre, in Vienna, October 1996.
Sub-task 4.2
The use of digital photogrammetric techniques for deriving information on
stand variability is being tested. Provisional results indicate that with 1:15
000 scale aerial photography and GPS ground control it is possible to derive an
elevation model with an RMS of 1.2 m in X/Y and 1.5m in Z. Data derived from
older, low oblique, photography is of a lower quality due, primarily, to the
low poor quality of the photographs.
At the conclusion of the field validation stage it will be possible to
quantify the accuracy of the technique. Methods for measuring stand variability
are being tested and field data will be used to assess their suitability and
success.
Continuation:
1992 and 1976 photos to be processed to produce orthophotos and DEMs
Currently have 1957 and 1995 DEMs and orthophotos for comparison (temporal)
Test orthos for each of 4 years against each other (x, y dimensions to test registration of images - x,y affects z error)
Test 1992 DEM against 1992 data
1976 photos include small trees - need to remove this effect
Derivation of tree heights through time using DEMs
Use of DEMs - check against yield models
- forest structure - thinning/species/felling/damage
- stand variability measures
- stand variability correlation with distance from stand edge, soil type, topography, aspect, slope, plough type, spacing
- amplitudes/frequencies - stocking density, thinning racks, surface roughness
Work to be completed - end of September 1997