Title: Applying Geospatial Technologies to Weed Mat Monitoring and Mapping: The Ythan Estuary, NE Scotland
1Applying Geospatial Technologies to Weed Mat
Monitoring and Mapping The Ythan Estuary, NE
Scotland
Annual Meeting 2003 GIS in Coastal and Marine
Areas Thursday 6th March 2002
- David R. Green Stephen D. King
- Centre for Marine and Coastal Zone Management
- University of Aberdeen, Scotland, UK
http//www.abdn.ac.uk/cmczm
2This presentation examines the potential for
using colour vertical aerial photography to map
the spatial location, distribution, extent, and
changes over time in weedmat coverage found in
the Ythan estuary, North East Scotland.
- Mapping of weedmats in the Ythan estuary has
previously been undertaken by Raffaelli et al. -
concluding that although fluctuating from year to
year there appears to be a gradual trend towards
an overall increase in the extent of weedmats
over time. - Using Raffaelli's work as a reference source the
objectives of the current work were to develop
an up-to-date practical methodology using image
processing and GIS (Geographic Information
Systems) software to provide information about
weedmat location and extent over time and to
provide quantitative estimates of weedmat
coverage for comparison with earlier studies. - Archival colour aerial photography for four years
(1989, 1992, 1994 and 2000) was scanned into a
computer. The application of image processing
techniques resulted in a series of mosaiced
images that were used as the basis for visual
interpretation and mapping of the location and
extent of weedmats for each year of photography. - Input of the maps into a Geographical Information
System (GIS) enabled the derivation of
quantitative estimates of the areal coverage of
macro-algal weedmats for each year and the
creation a series of maps showing change.
Introduction Summary
3- The Ythan estuary (57oN, 2oW) is the area of
study. It lies approximately 20 km to the north
of Aberdeen on the east coast of Scotland, and is
tidal extending for approximately 8km in the
general direction of Ellon. - The estuary averages about 300m in width, and has
an average depth at high water of 2.5m with tidal
range of between less than a meter to 3m or more
(Raffaelli, 1998, p. 138). - According to Raffaelli (1999, p. 164) The low
water channel is about 71 ha and there are 115 ha
of mudflat and 70ha of mussel beds and sand. - The Ythan drains a catchment of about 650 km2 of
intense arable agricultural land. The study area
chosen for this research is the entire intertidal
zone for the estuary.
Study Area
4- A significant visual feature of the Ythan estuary
in NE Scotland in recent years has been the
presence of benthic macro-algae (Enteromorpha,
Ulva and Chaetomorpha) forming extensive 'green'
'mats' over the estuary's intertidal flats. - One of the main reasons for the apparent observed
increase in the areal extent of macro-algal
weedmats is thought to be related to the amount
of nitrogenous fertiliser applied to the Ythan
catchment over time. - Studies in the Ythan estuary have been undertaken
to (a) identify the location and distribution
the weedmats (spanning a time period from the
late 1940s to the present day), and (b) try to
establish what is causing the weedmat. - The primary reason for undertaking such studies
is that the algal weedmats can have a dramatic
impact on the invertebrates in the underlying
sediment as well as on the shorebird populations.
Macro-Algal Weedmats
5 Report Preparation
I N P U T O U T P U T
Establish Background to Research Objectives and
Aims
Meeting 1
Study Area - Context and Setting Environmental
Problem - Defined Information Required Scene
Model of Phinn et al. (1999)
- Data Available
- Suitability for Task i.e. algal weedmat monitoring
Project Structure
Data Processing
- Manual Photo-interpretation
- Digitising
- Digital Image Pre-Processing
- GIS layers
Meeting 2
Analysis
Reports and Conclusions
Future Work
Meeting 3
6Geospatial Methodology
7- In order to study the location, distribution and
areal extent of macro-algal weedmats, over time,
a number of field survey- and aerial
photo-interpretation-based studies have been
undertaken to develop cost-effective
methodologies for repeat environmental data
acquisition. - The methods developed and used are well
documented in Raffaelli and Way (1996) and
include - the use of aerial photographs acquired from light
aircraft flying at 1000-1500 feet - photo-interpretation using vertical photographs
- the projection of colour photographic slide
transparencies for manual tracing and area
assessment with the aid of mm graph paper and a
planimeter (see e.g. Raffaelli and Plomer, 1989). - With developments in remote sensing (digital
image processing) and GIS software and hardware
technology the task of deriving spatial and
temporal information on macro-algal weedmats has
become far easier than was previously possible,
allowing for rapid data capture, storage,
handling and interpretation of remotely sensed
photography and imagery in a computer
environment. - There is considerable potential to (a) repeat
and check the results from previous studies, (b)
build up digital databases comprising
geo-rectified and mosaiced aerial photography and
vector map layers that can be used as a baseline
for future monitoring and mapping exercises, and
(c) derive quantitative information more quickly.
- Young et al. (2000, p. II-286) lend additional
support for a 'remote sensing approach' to
weedmat monitoring and mapping .'The rapid
development of benthic macroalgae on estuarine
mudflats during the summer growing season makes
difficult the accurate documentation of spatial
and temporal distributions of such algae from
ground surveys alone'.
Remote Sensing GIS
8The overall objective of the current work is to
devise a practical remote sensing-based
methodology to extract spatial information
(location and extent) about macro-algal weedmats
in the Ythan estuary derived from multi-temporal
archival aerial photography of varying different
scales acquired from different sources.
- More specifically
- aerial photo-interpretation of archival aerial
photography (panchromatic and colour) of varying
scales and dates obtained from local sources to
identify macro-algal weedmats - data capture of selected archival aerial
photography in the form of scanned raster digital
datasets - geo-correction of the scanned aerial photography
in a digital image processing system (Erdas
Imagine 8.4) - stitching or mosaicing of the geo-corrected
aerial photography (Erdas Imagine 8.4/PanaVue) - multi-temporal mapping of the location and areal
extent of macro-algal weedmats in the Ythan
estuary, Aberdeenshire, Scotland (Erdas Imagine
8.4 and ESRI ArcView 3.2) - comparisons between the information derived with
other, earlier results using different
methodologies to map from aerial photographs - creation and documentation of a series of
deliverables including digital aerial
photographs, interpretations, analyses, maps, and
other datasets
Objectives
9- Typically Enteromorpha forms vast 'mats' that lie
on the surface of estuarine mud- and sand-flats.
These mats can either be very fine coverings
(thin) or several cms deep (thick). - Although it is relatively easy to identify these
extremes on aerial photographs densities of cover
in-between are more difficult. - The water retention characteristics of the
species and also the presence of thin layers of
water covering the algae have been found to lead
to a reduction in the overall surface reflectance
making identification on photography and imagery
quite difficult - Infrared reflectance is usually high, and on CIR
photography the range of colours on the film
varies from a dull blue/grey when saturated (with
water) to deep red when present in thick healthy
'green' mats. - Where found in free-floating mats, Enteromorpha
may also be deposited on other vegetation types. - In a recent study around Portsmouth, Baily et al.
(2002) found that weedmats may form a crust of
dead material on the upper surface, leading to
similar reflectance characteristics to gravel
deposits. - Raffaelli et al. (1999, p. 108) note that In
situations like the Ythan estuary, Scotland,
Chaetomorpha is much more intimately associated
with the sediment than are Enteromorpha, Ulva, or
Cladophora, with substantial amounts of biomass
anchored firmly beneath the surface
Macro-algal Weedmats
10Photographic Coverage
1992
1989
2000
1994
11A small number of vertical 35mm colour aerial
photographs were taken in August 1994 using a
model aircraft. These are for part of the Sleek
of Tarty area only and do not cover the Ythan
catchment. They are large scale, cover a small
area and show macro-algal weedmats very clearly.
Model Aircraft Photography
12- The aerial photography selected, together with
any additional data and information (where deemed
appropriate), were transformed into a digital or
computer compatible format. - Conversion of the aerial photography from an
analogue to digital format makes it directly
compatible with digital OS map data and any other
GIS layers available from other sources e.g. the
sediment map from Raffaelli et al (1989) and
Stapleton and Pethick (1996). - Data capture of the analogue format aerial
photographs (verticals) was carried out using a
standard desktop scanner (Agfa Snapscan 1212).
Scanning at a resolution (X and Y direction) of
150dpi (colour) the photography was captured and
initially stored in a .TIF (F) format (Tagged
Image File Format). - Choice of the scanning resolution was primarily
based upon generating a filesize for scanned
photographs that was considered manageable (also
bearing in mind the need to subsequently mosaic
and geo-correct the images). Whilst the choice of
a high scanning resolution retains more of the
detail contained in the original document, a
compromise usually has to be struck in practice
between the relative gain in information content,
and the final filesize, as well as considerations
of filesize manageability for display,
transmission and storage.
Data Preparation
13Scanning
14- Ideally all photography should be geo-corrected
first to remove any inherent distortions if a map
is to be the end product. The result will be a
geometrically correct map (for each date) showing
the macro-algal weedmat distribution which can
then be used as a layer or coverage in
ArcInfo/ArcView 3.2. - Subsequently the digital raster aerial
photographs were input to the Erdas Imagine 8.4
Digital Image Processing and GIS software system
where they were - Geo-referenced (Projection/Spheroid/ Datum
Transverse Mercator/Airy/OS GB 1936). - Geo-referencing was achieved using digital OS
vector map data tiles for the designated study
area - Geo-referenced and mosaiced imagery were also
made available in the GeoTIFF format 'cropped'
using an on-screen digitised boundary (ArcView
3.2 shape file) of the Ythan estuary catchment
area
Geo-rectification
15All of the images selected for study were
geo-corrected using the Erdas Imagine 8.4 image
processing software referenced to OS digital map
data using between 4 and 16 GCPs per image, a
polynomial transformation (orders 1 and 2), and
resampled using nearest neighbour. Where
possible, the overall RMSE (Root Mean Square
Error) was reduced to lt1. One objective of this
work was to create a composite photographic image
for the entire estuary for each date of
photography available. To do this requires that
each geo-corrected aerial photograph be stitched
or mosaiced together. This was undertaken in
Erdas Imagine 8.4 using the Mosaic option. Whilst
some results were good, others resulted in
relatively poor colour balancing between each
photograph making up the mosaic. Subsequently,
mosaicing was also tested in a software product
PanaVue (http//www.panavue.com) with markedly
improved colour balancing results.
Geocorrection Mosaic
16Mosaic
17- In order to conserve computer disk storage
requirements for the digital aerial imagery, and
to speed up the display of the backdrop for the
purposes of digitising, a digital vector map
outline of the Ythan estuary high water mark
(HWM) (captured as an ESRI .shp (Shape file
format) via on-screen digitising using ArcView
3.2) was used to define an image 'cookie cutter'
delimiting the boundary of the Ythan estuary
study area. - The filesizes of the resulting raster datasets
were significantly reduced. However, in practice
it was generally found that the full image mosaic
provided more informational 'context' for
interpretation.
Filesize
18- Default color balancing of the mosaiced image
files completed in Erdas Imagine 8.4 was
generally deemed to be satisfactory, although not
all yielded good results due to the differences
between individual images making up the mosaic
(arising from the sun-sensor-ground angles,
sensor view angle, and possibly small atmospheric
changes or differences, the camera, and
photographic processing). - The mismatch in colour between one photograph and
another is, for the most part, not a serious
problem for mapping the weedmats, although in a
few cases the resulting image colour balance
would benefit from closer matching. - Further investigation of this potential visual
interpretation problem revealed the availability
of some other mosaicing software (PanaVue from
Canada) that helps to markedly improve the colour
balancing between individual photographs and the
resulting mosaic. PanaVue also allows manual
positioning of the individual photographs making
up the mosaic.
Colour Balancing
19- Each cropped mosaic image of the Ythan estuary
was input to the ArcView 3.2 GIS software package
in Imagine (.IMG) format. Units of macro-algal
weedmatting were mapped using the online
interactive digitising tools. - Consistency in the interpretation (drawing
boundaries and identifying the category) is
important, hence the work was carried out by a
single individual to (a) digitise, and (b)
interpret. - The minimum mappable unit (mmu) is often used as
the basis for determining a 'cut-off' point or
threshold for deciding which map units to
digitise and which not to digitize. - To some extent the choice of unit to map also
depends upon - original photographic scale
- quality of the aerial photography/mosaic
- degradation of the photographic image e.g. due to
scanning/screen display - image contrast and sharpness between the feature
of interest and the background or surrounding
area/surface (sometimes difficult towards the
edges of aerial photographs) - Image magnification
- Interactive zooming (in and out of an image)
- eyesight of the interpreter
- experience of the interpreter
- quality of the display/original (electronic
colour versus reflective colour) - Some assistance in the interpretation exercise
can also be offered through (a) reference to
contextual information and (b) an examination of
the hardcopy aerial photographs
Interpretation
20- In general, weedmats appear to be a distinct
'emerald green' colour in contrast to the 'pale
brown/greybrown' colour of the 'sand/mud'
background. Some, however, appear 'brownish' in
colour in the digital imagery and visual checks
had to be made with the original photographic
prints. - The general location of the weedmats in each
mosaic also seem to correspond to the 'habitat
conditions' e.g. sheltered marine environments,
middle to low intertidal zone, calm, protected
harbours etc. - Overall the weedmats do appear to show a strong
visual contrast with the bottom sediments (sand
and mud). Only where they overlap darker bottom
sediments e.g. mussel beds/sea weed does it pose
difficulty in accurately delineating weedmat
units (e.g. where they overlap). Some help is,
however, provided by examining a sediment map for
the area by Stapleton and Pethick (1996).
Photo-interpretation Key
21In order to narrow down the decision-making
process (weedmat/not weedmat) contextual
information (e.g. text sources, maps etc.) can be
added into the mapped layers when digitising the
weedmat boundaries. For example, a bottom
sediment or substrate map showing sand, mud,
mussel beds and other known 'habitat' factors.
Contextual Information
22Change Detection
23Spectral Profiles
24'On-screen' digitising of features on aerial
photography can be undertaken directly onto a
single or mosaiced image backdrop displayed in
Arc/Info, ArcView 3.2 or Erdas Imagine 8.4.
Accompanying each digitised map layer in
ArcView 3.2 is an attribute table comprising the
attributes of the feature type e.g. polygon/area,
and an ID number for each algal weedmat unit
(polygon) digitised, together with any other
information associated with the polygon.
ArcView 3.2 GIS
25- Originally it was anticipated that the weedmat
maps derived by Raffaelli et al. for the Ythan
estuary would all be made available for this
work. Using the display and overlay functionality
of a GIS it would be possible to undertake the
following - check the quantitative areal measurements of
weedmat coverage for the Ythan estuary as
obtained by Raffaelli et al. and as shown on the
accompanying maps and histograms - overlay weedmat maps for the same year obtained
from different sources (Raffaelli et al. Green
and King) to compare and contrast both the
interpretations and the quantitative estimates of
weedmat cover - Unfortunately only one map for 1986 was
available. It was a very generalised paper
hardcopy black and white map, and had been
derived from oblique colour photographic slide
transparencies of the Ythan estuary.
Some Problems
26An alternative approach, therefore, was to
consider some of the weedmat maps made by
Raffaelli et al. for the South Quay area at
Newburgh (other areas are also shown in Raffaelli
and Way, 1996). Although these were similarly
rather generalised maps they provided one way of
'comparing and contrasting' interpretations and
quantitative data. There are, however, some
limitations to this approach that must be
considered when interpreting the results.
- the maps sourced from Raffaelli are of a very
small format, generalised and appear to be the
result of work by several different interpreters - comparison of the temporal maps through use of
overlay techniques reveals that they are indeed
quite generalised and moreover they do not
entirely co-register or have the same locational
reference points - other sources of error may have arisen in the
derivation of the original maps e.g. different
interpreters, different scales of photography. It
is more than likely that all the maps contain
errors, but it is difficult to assess this
quantitatively or to be able to report it easily - inspection of the additional reports by Raffaelli
et al. (1988, 1989, 1996) reveals some useful
information about the areal extent of weed mats
as measured on aerial photography taken at
different points during the season. - Raffelli and Plomer (1989, p.1) also indicate
that the 1989 survey was carried out two weeks
later than in previous years resulting in the
weed mat being past its peak. - the fact that the survey of 1986 was mapped from
50 colour transparencies via projection onto
paper (Raffaelli and Nicol, 1988) suggests some
possible sources of inaccuracy as far as the
mapping and weedmat area estimates are concerned.
Comparisons
27However, bearing these limitations in mind it is
possible to locate four ground control points on
each map that appear to be the 'same' and to
geo-correct each of the maps, sufficient to
facilitate overlay with the geo-corrected aerial
photographs and mosaics, as well as with the maps
for each date. This allows some comparisons to be
made between different years and potentially
forms a basis within the GIS for showing change
over time that can then be mapped as a 'change'
map (using different colours and shadings) for
each year, or to be animated. Reference to the
photographic interpretations of the weedmats does
reveal, however, that
- spatial location of the weedmats is often similar
for each year - there are definite differences in areal weedmat
coverage from year to year (as might be expected) - comparisons between the different interpretations
available show that strong similarities exist but
that differences do arise possibly due to the
interpreter's choice of the minimum mappable unit
(mmu), the accuracy of the rectified imagery
being used to map from, the weedmat units which
are identified and drawn around (greater or
lesser detail), and the details of the boundaries
drawn (see Green and Hartley, 2000). This
suggests that quantitative measurements of the
areal weedmat coverage - in terms of how many
metres squared were present at each date - may be
larger or smaller than they really are at any one
time, or alternatively may be less than they
really are due to the time of year that the
photography was acquired and the interpretation.
Comparison
28- The input of geo-referenced datasets to ArcView
3.2 makes it possible to undertake some - simple GIS analyses to
- show the spatial distribution and pattern of
algal weedmats over time (overlay analysis) - show the changes in the areal distribution of the
weedmats between e.g. time 1 and time 2 (T1-T2 -
a simple form of change detection or
differencing) - estimate the total areal coverage of weedmats in
metres squared for each date - estimate the percentage change over time
An ArcView extension, XTools, was downloaded and
installed to select polygons in a theme or
coverage that has geographic co-ordinates and to
derive e.g. area measurements in metres squared,
acres or hectares for each polygon. It is also
possible to make use of the Swipe, Blend and Fade
tools in the Erdas Imagine 8.4 Viewer to provide
a visual basis for observing changes in weedmat
distribution, pattern and coverage over time. A
number of different maps using the View, Layout
options can be derived for the Ythan estuary from
the environmental map and image databases created.
GIS Map Output
- Photomosaic and Vector Overlay
- Weedmat Coverage Maps for each year
- Composite Weedmat Coverage Maps
- Classes of Weedmat Coverage for each year
291989
1992
GIS Map Output
2000
1994
301989
1992
GIS Map Output
1994
2000
31Results
32Photo-interpretation experience suggests that if
aerial photography were to be flown again, for
repeat monitoring and mapping of weedmats ideally
it is
- best to acquire either high quality large scale
vertical colour aerial photographs at a scale of
13,000-15,000, or high quality large scale
vertical colour infrared (CIR) aerial photographs
at a scale of 13,000-15,000 in a 9"x9" format
preferably with fiducial marks and a GPS (note
other examples of work on the use of remote
sensing for weedmats have made use of scales of
17,200, 18,000, 110,000, 112,000 and 116,666)
In both cases the high spatial resolution of the
photography enables an interpreter to(a) both
see and locate the algal weed-mats because of the
high contrast between the 'vegetation' and the
substrate (mudflats, sandflats) and (b) map
the boundaries of the identified units with
better accuracy
Recommendations
It is suggested that if more aerial photography
is to be acquired in the future for an extension
of this work that
- For comparisons with previous aerial
photographically derived data and information
that it is acquired at approximately the same
time as in previous overflights using the same
camera setup and film at the same scale - Some aerial overflights are also made for other
times during the season to try to ascertain
whether or not there is an optimum period for
aerial photographic acquisition to assist in the
identification and mapping process. The optimum
overflights could (within reason bearing in mind
aircraft/pilot availability, prevailing weather
conditions, cloud cover, etc.) be established by
examining the previous aerial photography
acquired and other information sources and
graphs such as provided by Raffaelli et al.
(1999, p. 108).
33 - Colour infrared (CIR) and/or multispectral aerial
photography is considered in the future for such
studies, the former offering the opportunity to
maximise the identification and separability of
the macro-algal communities from the substrate.
Young et al. (1999) also suggest the use of
colour negative film. - Other sources of remotely sensed imagery are
considered as alternatives to aerial photography
that potentially offer a similar (or better
spatial resolution) and better spectral
resolution - It would be highly appropriate to gather some
field or ground-based data and information
coincident with the aerial photography to assist
in characterising the photographic appearance of
the macro-algal weedmats for different localities
and densities (see for example work by Young et
al. (1998))
Recommendations
34- Future Studies are now underway to examine the
problem of weed mat monitoring and mapping in the
Ythan
- As part of this current work due account has also
been taken of the long-term requirements of the
Ythan Project - The catalogue of datasets used for this project
provides a useful reference and record for any
future work and for any further comparisons
between digital datasets - The digital datasets generated also form a useful
resource and baseline database for the Ythan
estuary, which could eventually form the basis
for a GIS for the Ythan - Subsequently the map datasets could also be
utilised within an online mapserver e.g. ArcIMS
to provide a means for provision of public access
to environmental information about the Ythan
estuary and its surrounds - This work has also provided a basis upon which it
is now possible to compare and contrast the work
already undertaken by Raffaelli et al. on the
Ythan by creating new weedmat map
interpretations for comparison with the original
maps and aerial photographs and for digital
capture of the Raffaelli et al. datasets for
integrated analysis and comparison with new
interpretations - Compatibility of the geo-corrected photographic
datasets (single and mosaiced) with other digital
GIS datasets (e.g. SNH and other data) will also
be useful for future GIS analyses - The practical methodology developed here will
also be useful for any subsequent comparisons
with information derived from other remotely
sensed image sources e.g. CASI, ATM and LIDAR.
Additionally, the information derived can also be
analysed within a GIS. LIDAR data, for example,
offers the opportunity to acquire digital height
data and to derive a Digital Elevation Model
(DEM).
Future Studies
35- This may subsequently be useful for image
display, interpretation and classification,
particularly if a 3-D terrain model with imagery
draped over the top has value for visualisation
purposes in the public domain. - Assuming that one of the long-term goals of this
work is to generate information about weedmat
coverage and extent over time, it may be
appropriate to consider the need to develop a
similar or compatible methodology for mapping
weedmats from different sources of remotely
sensed data/imagery that may be compared with the
work done here and by Raffaelli et al. This will
be particularly important to consider if any
observations are to be made about spatial pattern
and distribution over time as well as for
quantitative change detection. - Based upon the information provided in the
documentation available, analysis of CASI imagery
and LIDAR data may also be used in future studies
to determine the role that they (remote sensing
and heighting data respectively) can play in
monitoring, identifying, and mapping macro-algal
weedmats. CASI imagery, for example, offers finer
multi-spectral resolution than aerial photography
that may be of help in the identification of the
macro-algal weedmats. Similarly with Airborne
Thematic Mapper (ATM) which has successfully been
used by NERC for macro-algal weedmat monitoring
and mapping in the south of England. - The application of digital image processing of
aerial photography is not straightforward and
classification of the weedmats versus the
surroundings is not always successful and
repeatable. This is a view similar to that of
Young et al. (1999, p.1176) who observed that
'the use of digital image processing techniques
alone to classify. was problematic'.. because
macroalgae, for example, share the spectral
characteristics of other classes in the visual
portion of the electromagnetic spectrum.
Alternative approaches might be to utilise
Definien's ECognition software that incorporates
contextual information into the spectral
classification. A solution proposed by Young et
al. (1999, p. 1176) is 'to use these techniques
as a first step in the classification process,
followed by a visual examination of the imagery,
and to edit the results in a traditional approach
to photointerpretation'.
Future Studies
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References
37Thank You
In conclusion, the combination of digital data
capture, conversion and input capabilities,
digital image processing and GIS technology
offers considerable potential for integrating
archival map and image datasets, as well as
providing the potential to create a baseline
GIS-based system as the basis for future
multi-temporal monitoring and mapping of
environmental change of macro-algal weedmats in
the Ythan.
- David R. Green Stephen D.King
- Centre for Marine and Coastal Zone Management
- Department of Geography and Environment
- School of Resources, Environment and Society
- University of Aberdeen, Elphinstone Road
- Aberdeen, AB24 3UF, Scotland, UK
- Tel. 44 (0)1224 272324
- Fax. 44 (0)1224 272331
- Email. d.r.green_at_abdn.ac.uk / s.d.king_at_abdn.ac.uk
- Internet. http//www.abdn.ac.uk/cmczm