Charlton, M.

Coveney, S.

Fotheringham, S.

Sweeney, J.

English

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, 
 
 
 
 
 
 
 
 
 
Coveney, S., Fotheringham, S., Charlton, M., and Sweeney , 
J. (2007) Dual-frequency GPS survey for validation of a regional DTM and 
for the generation of local DTM data for sea-level rise modelling in an 
estuarine salt marsh. In: 9th International Conference on Geocomputation, 
3-5 Sept 2007, NUI Maynooth, Ireland. 
 
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Dual-frequency GPS survey for validation of a regional 
DTM and for the generation of local DTM data for sea-
level rise modelling in an estuarine Salt Marsh. 
 
Seamus Coveney
1
, Stewart Fotheringham
2
, Martin Charlton
3
, John Sweeney
4
 
1National Centre for Geocomputation NUIM, Ireland & Department of Geography, NUIM, Ireland. 
Telephone: (00353-1-7086196)  
Fax: (00353-1-7086456) 
Email: seamus.j.coveney@nuim.ie 
 
2 & 3 National Centre for Geocomputation NUIM, Ireland. 
2 Telephone: (00353-1-7086455)  3 Telephone: (00353-1-7086186) 
Fax: (00353-1-7086456)  
2 Email: stewart.fotheringham@nuim.ie  3 Email: martin.charlton@nuim.ie 
 
4 Department of Geography, NUIM, Ireland. 
Telephone: (00353-1-7083684)  
Fax: (00353-1-7083573) 
Email: john.sweeney@nuim.ie 
 
1. Background 
Global average temperatures have risen by an average of 0.07°C per decade over the last 
100 years, with a warming trend of 0.13°C per decade over the last 50 years. 
Temperatures are predicted to rise by 2°C - 4.4°C by 2100 leading to global average sea-
level rise (SLR) of 2 – 6mm per year (20 – 60cms in total) up to 2100 (IPCC 2007) with 
impacts for protected coastal habitats in Ireland. 
Estuaries are predominantly sedimentary environments, and are characterised by shallow 
coastal slope gradients, making them sensitive to even modest changes in sea-level. The 
Shannon estuary is the largest river estuary in Ireland and is designated as a Special Area 
of Conservation (SAC) under the EU Habitats Directive (EU 1992) providing protection 
for listed habitats within it, including estuarine salt marsh. 
Trends in Shannon estuary tidal data from 1877 – 2004 suggest an average upward SLR 
trend of 4 - 5mm/yr over this period. A simple linear extension of this historical trend 
would imply that local SLR will be in the region of 40 - 45cm by 2100. However, this 
may underestimate actual SLR for the estuary by 2100, since it takes no account of 
predicted climate-driven global SLR acceleration (IPCC 2007) up to 2100. 
2. Sea-level rise modelling in GIS 
Much work has been done to model SLR in Geographic Information Systems (GIS) 
(Titus & Richman 2001, Hennecke & Cowell 2000) and additional work has been carried 
out to assess the significance of DTM error on SLR predictions (Cowell & Zeng 2003, 
Blomgren 1999). Opportunities exist however to consider DTM quality for Ireland and to 
develop reliable SLR prediction methods in an Irish context. The sensitivity of the 
Shannon estuary to SLR and its designation as a habitat conservation area suggests that it 
is a suitable location to develop techniques supporting reliable SLR modelling for coastal 
conservation in Ireland. 
3. Scope of Shannon assessment 
This paper discusses some of the questions that have been explored in terms of refining 
the accuracy of sea-level rise modelling in a salt marsh complex in the region of Shannon 
town. Predictions are based on the observed tidal data, and utilise Intergovernmental 
Panel on Climate Change (IPCC) predictions (IPCC, 2007) for global SLR up to 2100. 
The significance of regional post-glacial land-surface rebound (Devoy, 2000) predicted 
SLR acceleration (IPCC, 2007) and local deposition trends are also considered. 
The generation of meaningful SLR prediction maps in a Geographic Information System 
(GIS) presupposes the existence of highly accurate Digital Terrain Model (DTM) data. 
The Ordnance Survey Ireland (OSi) 1:50,000-scale spot height and contours dataset 
provides the basis of the national coverage DTM dataset for Ireland. However, vertical 
errors in this dataset are quoted at ±5m, suggesting that it is unsuitable for SLR 
modelling. OSi vector mapping at larger scales does however provide some additional 
height-encoded coastal data that might be used to augment the national coverage DTM at 
the coastline. This question is examined using dual-frequency (i.e. survey-grade) Global 
Positioning Systems (GPS) survey. The OSi DTM for Ireland is currently being upgraded 
with aerial ‘LIght Detection And Ranging (LIDAR) survey data (characterised by vertical 
errors of ±15cm) but coverage is as yet limited to major urban areas, and plans to extend 
surveys beyond these areas are at an early stage. Separate aerial LIDAR surveys of the 
coastline have also commenced, under the auspices of the Irish Department of the 
Marine, but as yet these surveys have not extended to include the Shannon estuary. The 
need for high-quality DTM for effective SLR modelling, coupled with the lack of 
accurate DTM data for the Shannon estuary study area, created a need for a newly 
generated local DTM from first principles. 
3. Methods 
Dual-frequency GPS survey data is capable of defining elevation to a precision of 1 – 
2cm relative to the local OSGM02 geoid when post-processed against OSi GPS reference 
station data. Dual-frequency GPS survey data was collected for a salt marsh complex 
located to the south of Shannon town and post-processed against OSi GPS refence station 
data from the OSi GPS reference station at Limerick University. The results of this 
survey were compared with the national coverage DTM to quantify and classify the error 
ranges encountered within the OSi dataset, and to provide a high-quality point dataset for 
the generation of a local DTM for SLR prediction in GIS. The spatial extent of the GPS 
survey area was extended beyond the limits of the salt marsh complex to cover a range of 
OSi spot height and contour elevation values for the error assessment. Extension of the 
GPS point survey also offered an opportunity to map elevation in urban coastal areas 
landward of local coastal defence barriers. The area defined for these assessments was 
sub-divided into an initial network of 250 sub-blocks, measuring 100x100m, providing a 
sampling framework for the dual-frequency GPS survey. A Trimble R8 dual-frequency 
differential GPS receiver was used to capture 250 x,y,z coordinate points relative to the 
Irish WGS84/ETRS89 Ellipsoid and the local OSGM02 geoid at an average horizontal 
separation of 80m. GPS points were recorded in fast-static mode, which provides better 
elevation accuracies than is currently possible with the (beta version) OSi real-time GPS 
network. Trimble Geomatics Office (TGO) software v.1.62 was used to post-process the 
survey points against Receiver Independent Exchange Format (RINEX) data from the 
OSi GPS reference station at Limerick University. The average vertical error in the GPS 
captured points after post-processing was 11mm, with a maximum vertical error of 
35mm, and a minimum error of 4mm. Locations that corresponded with points on the OSi 
spot height and contour datasets were identified and surveyed with the Trimble R8. The 
difference between the GPS and OSi elevations at the spot heights and points sampled 
along the OSi 10m and zero contours were broadly within published OSi ranges. The 
vertical accuracies achieved across the entire GPS survey area suggest that dual-
frequency GPS is a reliable source of base data for the generation of a DTM of sufficient 
quality for use in SLR prediction mapping. 
It was possible to evaluate the accuracy of the OSi coastline definition by reference to 
GPS measurements relative to the geoid locally, but systematic vertical separation 
between the OSi definition of zero elevation and the measured elevations was observed. 
Local tidal regimes and a range of diverse definitions of zero are likely to account for the 
systematic separation observed (different methods typically include Mean tide mark, 
mean high-water mark, and highest astronomical tide mark). An additional problem 
exists however with definition of the absolute coastline using GPS. Collection of a 
network of elevation-encoded points is extremely accurate, but definition of a coastline 
using GIS would have required substantial offshore GPS surveying. This difficulty can be 
overcome with the use of Terrestrial LIDAR survey. 
4. Additional work planned 
Terrestrial LIDAR offers even greater scope for the generation of a high-quality local 
DTM data, typically returning vertical accuracies of less than 2cm, at an average 
horizontal ground-sampling density of better than 10cm. Terrestrial LIDAR surveying 
will be carried out using a Leica Scanstation instrument in a sub-section of the GPS 
survey area. This approach is expected to overcome difficulties in the definition of the 
salt marsh edge, and will provide a substantially more detailed DTM than the GPS 
survey. Final DTM products will be used as an input into a multi-variable model of salt 
marsh change in GIS. 
A crucial question alluded to above is the degree to which local deposition rates might 
affect observed SLR in the estuary. Field observations suggest that sections of the salt 
marsh complex may be experiencing accretion, while other locations appear to be subject 
to erosion. In addition, the GPS survey reveals that salt marsh areas seaward of the 
reclamation barrier are approximately 1m higher than the land surface behind the 
reclamation barrier, suggesting that that existing salt marsh on the seaward side of the 
barrier has experienced significant build-up since the barrier was put in place 
approximately 100 years ago (dates derived from map and documentary sources). A 
network of 35 silt measurement pans was set up at intervals of approximately 500m along 
an 8km stretch of coast in the study area. Initial silt deposition survey results suggest 
variable rates of up deposition of 0 - 60mm/yr, suggesting geographical variation of 
erosion and deposition.  
The final results of the silt measurement information (a six month period of measurement 
is half completed) will be used as reference data for the generation of a multi-variable 
grid model of salt marsh change in GIS. This model will attempt to predict the likelihood 
of salt marsh occurrence with SLR up to 2100, based on elevation and slope (as defined 
by the locally generated DTM) in addition to grid file definitions of coastal energy 
gradient and erosion/deposition ArcGIS Spatial Analyst. 
The effectiveness of using the DTM-derived variables and the energy gradient model as 
predictors for salt marsh occurrence will be tested on existing marsh areas. Based on the 
success of this method, cell-by-cell predictions of possible salt marsh change up to 2100 
will be postulated. Management recommendations will be made based on these results. 
5. Acknowledgements  
This research is funded entirely by the National Centre for Geocomputation at the 
National University of Ireland at Maynooth. Base Ordnance Survey Ireland digital data 
utilised in this project is provided under an OSi sub-contract agreement on the OSi digital 
licence for Clare county council. 
6. References 
Blomgren S, 1999, A digital elevation model for estimating flooding scenarios at the Falsterbo Peninsula, 
Environmental Modelling & Software, 14, 579–587. 
Cowell PJ and Zeng TQ, 2003, Integrating Uncertainty Theories with GIS for Modeling Coastal Hazards of 
Climate Change, Marine Geodesy, 26, 5-18. 
Devoy RJN, 2000, Implications of Sea-level rise for Ireland. in, R. J. Nichol and A. de la Vega (eds.), 
Vulnerability and Adaptation to Impacts of Accelerated Sea-level Rise, pp. 33-46. Proceedings of 
the SURVAS Expert Workshop, ZMK University of Hamburg, 19-21 June 2000. EU Concerted 
Action Programme, Brussels. 
EU, 1979, Council Directive (79/409/EEC), on the conservation of wild birds. Office for Official 
Publications of the European Communities. 
EU, 1992, Council Directive 92/43/EEC; on the conservation of natural habitats and of wild fauna and 
flora. Office for Official Publications of the European Communities. 
Hennecke & Cowell 2000, GIS modelling of impacts of an accelerated rate of sea-level rise on coastal 
inlets and deeply embayed shorelines, Environmental Geosciences, 7(3): 137-148 
IPCC, 2007 Climate Change 2007: The Physical Science Basis, Summary for Policymakers. Contribution 
of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate 
Change, Intergovernmental Panel on Climate Change. 
Titus & Richman 2001, Maps of lands vulnerable to sea level rise: modelled elevations along the US 
Atlantic and Gulf coasts, Climate Research, 18, 205–228. 


Dual-frequency GPS survey for validation of a regional DTM and for the generation of local DTM data for sea-level rise modelling in an estuarine salt marsh

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Dual-frequency GPS survey for validation of a regional DTM and for the generation of local DTM data for sea-level rise modelling in an estuarine salt marsh

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