Revista de Gestão Costeira Integrada
Volume 8, Número 1, 2008, Páginas 9-23

DOI: 10.5894/rgci27
* Submissão - 20 Novembro 2007; Avaliação e Decisão - 8 Janeiro 2008; Recepção da versão revista - 9 Fevereiro 2008; Aceitação - em 12 Fevereiro 2008; Disponibilização on-line - 11 de Março 2008

Aplicação de metodologias de monitorização GPS em litorais arenosos: Geração de modelos de elevação do terreno *

Application of GPS survey methodologies in sandy shore environments: Generation of digital elevation models

Paulo Baptista ¹, Luísa Bastos ¹, Telmo Cunha ²,
Cristina Bernardes ³, João Alveirinho Dias ⁴

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1 - Autor correspondente - renato.baptista@fc.up.pt, Observatório Astronómico – Faculdade de Ciências – Universidade do Porto, Alameda do Monte da Virgem, 4430-146 Vila Nova de Gaia, Portugal, Phone: +351 227861290; Fax: +351 227861299
2 - Instituto de Telecomunicações – Universidade de Aveiro, Campus de Santiago, 3810 Aveiro, Portugal
3 - Departamento de Geociências - Universidade de Aveiro, Campus de Santiago, 3810 Aveiro, Portugal
4 - FCMA/CIMA – Universidade do Algarve, Campus de Gambelas, 8005-139 Faro, Portugal

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RESUMO
A rapidez no processo de monitorização de segmentos litorais constituídos por praias arenosas relativamente extensas, aliada a níveis de precisão indispensáveis para estudos de evolução da linha de costa e de caracterização morfodinâmica implica, frequentemente, o desenvolvimento de novas metodologias. Durante as últimas décadas, os levantamentos de campo foram efectuados recorrendo, principalmente, a técnicas fotogramétricas ou de geodesia clássica. Com o advento das técnicas de geodesia espacial, novas metodologias de base terrestre e aérea foram introduzidas nos programas de monitorização litoral.
O presente trabalho apresenta um protótipo que foi desenvolvido para monitorizar, de forma eficiente e produtiva, as morfologias presentes em praias arenosas. Este protótipo recorre à utilização do Sistema de Posicionamento Global (= “Global Positioning System” – GPS), incorporando um sistema multi-antena GPS montado numa plataforma móvel - um veículo todo-o-terreno apropriado para se deslocar em ambiente de praia. A monitorização é efectuada sob a forma de uma rede de perfis longitudinais e transversais relativamente à linha de costa, que incluem toda a praia sub-aérea, desde o limite de espraio da onda até à base do cordão dunar frontal, ou outra variação topográfica significativa do terreno. A partir da rede de perfis GPS obtida, cuja densidade é definida, em cada caso, de acordo com as características do terreno, é gerado um modelo de elevação do terreno (= “Digital Elevation Model” – DEM). A partir desse DEM é possível extrair informação relevante para caracterização morfodinâmica de praias arenosas, nomeadamente o volume da praia emersa, o declive da face da praia, a localização de bermas e de outros elementos morfológicos.
No que diz respeito à eficácia da metodologia proposta, os testes de validação que foram realizados permitem concluir que a precisão final dos DEMs é superior a 0,10 m (RMS) (valores médios de RMS entre 0,07 e 0,09 m). Relativamente à produtividade alcançada verifica-se que esta é dependente, essencialmente, do estado morfodinâmico da praia. Em praias dissipativas, a experiência adquirida permite constatar que é possível monitorizar, em média, três quilómetros de praia por hora, considerando uma largura média de praia de cerca de 150 m. Estes valores decrescem para cerca de um quilómetro de praia por hora quando se trata de praias reflectivas, com terraço de maré, cúspides bem desenvolvidas, e uma ou mais bermas na média e alta praia.
A presente metolodogia pode considerar-se promissora para a realização de programas regulares de monitorização de baixo custo, oferecendo vantagens adicionais de independência relativamente às condições meteorológicas, permitindo, por conseguinte, a caracterização do impacte de temporais.

ABSTRACT
The development of survey systems characterised by high accuracy and productivity is fundamental when it is intended to establish regular monitoring programmes in large littoral stretches. The acquired data is fundamental to study the shoreline evolution trends and to support other morphodynamic studies.
During the last decades, the general methodological approach for the establishment of coastal monitoring programmes was essentially based on photogrammetry or classical geodetic techniques. With the advent of new geodetic techniques, space based and airborne based, new methodologies were introduced in coastal monitoring programmes. The use of the Global Positioning System (GPS) to support land based sandy beach studies started in the 1990’s. In most of the works cited in the scientific literature the GPS antenna is adapted in a land vehicle or transported on the top of a surveying pole by the operator. Others applications include the use of GPS integrated with active sensors in aerial platforms, like the Airborne Laser Scanning (ALS), known by the acronym LIDAR (LIght Detection and Ranging). These technique allow high productivity and accuracy in sandy shore survey, with additional advantages in relation to photogrammetry related with the possibility of generate Digital Elevation Models (DEMs). However, the high cost of these systems limits the generalisation of its application in sandy shore environments.
Most of the errors that affect GPS techniques when land based, on-foot methodologies are employed, are related with careless operation during the survey. Heterogeneous burying and inclination of the telescopic pole where the GPS antenna is installed induce random errors that can reach several tens of centimetres. In the case of kinematic GPS surveys where the telescopic pole is carried on-hand, without a contact point with the ground, it is difficult to assure a constant distance in relation to the ground surface. When the kinematic GPS antennas are installed in land vehicles, systematic position errors can be committed due to ground slope changes. These errors affect the positioning precision, but can be compensated if more than one GPS antenna is used in the vehicle.
This paper concerns with the development of a monitoring prototype, to survey the sandy shore morphologies, which is based in the use of the GPS. This prototype has a multi-antenna GPS based system mounted on a fast surveying platform, a land-vehicle appropriate for driving in the sand (four-wheel quad). This system was conceived to perform a network of profiles in sandy shores stretches, since the swash line until the frontal dune baseline (sub-aereal beach), in littoral stretches with several kilometres of extension. From the acquired data high precision Digital Elevation Models (DEMs) can be generated. From these models, it is possible to compute the sedimentary volume present in the sub-aereal beach, the beach face slope, to locate the sand cusps, berms, the frontal dune baseline and other morphological elements present in sandy shores.
An analysis of the accuracy and precision of some Differential GPS (DGPS) kinematic methodologies is presented. The development of an adequate survey methodology is the first step in the morphodynamic shore characterisation or in the coastal hazard assessment. The sampling method and the computational interpolation procedures are important steps to produce reliable 3D surface maps as close as possible to the reality. The quality of several interpolation methods used to generate grids was tested in the areas where there were data gaps. The results obtained allow to conclude that with the developed survey methodology, it is possible to survey sandy shores stretches, under spatial scales of kilometres, with vertical accuracy in the final Digital Elevation Models (DEMs) higher than 0,10 m (RMS). This precision is slightly better than the precision cited in the literature for the DEMs obtained from ALS when applied in shore context. After validation with independent techniques the precision of a DEM obtained by ALS is situated between 0.15 and 0.25 m (RMS) in the vertical component.
The productivity of the proposed methodology depends on the beach morphodynaic state. From previous experience in dissipative beaches it can be concluded that it is possible to survey littoral stretches of about three kilometres extension per hour. In reflective beaches these values decrease to one kilometre per hour.
Since 2002 several study cases have been conducted in Portuguese west coast, which include exposed, semi-exposed and protected sandy beaches. These studies have been carried out on regular seasonal bases that allow a short-term morphodynamic characterisation.
The present methodology can be considered promising since it allows to perform relatively low cost regular monitoring programmes, with additional advantages regarding the possibility of surveying in almost all meteorological conditions, that is of fundamental importance to analyse the storms impact over sandy shores.

 

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