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Long-term mean sea level measurements along the Brazilian coast333to a cable and one pulley. When the cable isdisplaced on the pulley, an encoder transforms thismovement into a measurement of sea leveloscillation. Despite the equipment being simple toinstall and widely used, it is susceptible to manyerrors, such as sedimentation in the installation site,biological encrustations and crustal movements. Theradar tide gauge is a new technology recentlyapplied in Brazil. The main advantage of this systemis its installation (it is located outside the water) andthus, is not susceptible to temperature and densityoscillations. It measures the distance between theair–sea interface and the equipment through anacoustic signal. However, a major disadvantage ofthis system is the energy demand in case of use inlong-term research.Protocols for measuring MSL using tide gaugesSince tide gauges are being currently used inBrazil to measure MSL, we will further discuss theprotocols associated with this technique.The choice of system to measure sea leveldepends mostly on the purpose for which the data isto be used. Aspects such as costs, accuracy, locationand duration of measurements must be taken intoaccount to make the best decision possible. Forexample, harbor operations demand an accuracy ofabout 0.1 m (Pugh 2004). Hence, there is no need ofthe equipment to be sophisticated and a low cost tidegauge may be used accordingly.Scientific studies focusing on MSL, on theother hand, require a more accurate estimate ofabout 0.01 m. In this case, the installation must havean adequate number of reference levels (RRNNs) tomonitor possible changes in the ruler(s) position, tocarry out a topographic–geodesic monitoring of thereference levels and tide gauges, besides a digitaldata record (Fig. 1). These items will be discussedlater.Nevertheless, once the choice is made, it isessential to follow its basic recommendationsaccordingly to obtain a valid and usefulmeasurement.The use of metric ruler is the simplest wayto measure sea level. Despite being quite susceptibleto positioning mistakes, it is still used as acalibration reference to verify and/or correcteventual vertical displacements of already installedtide gauges.Tide gauges are relatively simple to install,widely known and do not require sophisticatedtechnology to operate. However, there are manyerrors and some disadvantages associated with them.For example, they are for local use (i.e., theinformation is restricted to the point being sampled),are subject to operational errors (e.g., biologicalincrustation), crustal movements, meteorologicalfactors, geographic positioning of the levels’references, etc.There are many types of tide gauges. Theycan be classified, for example, according to thephysics employed to obtain the information. Thereaders can refer to UNESCO (2002) for a completelist of tide gauges and their operating systems. Toensure accuracy of results, all of them commonlyfollow the same basic installation requirements. Anymeasurements of height should have a referencelevel relative to that plane. A reference plane is welldefined locally on a stable surface, free of anyinfluence of vertical and horizontal movements,Figure 1. Scheme of the system of measurement using a tide gauge. Adapted from UNESCO (2002).Pan-American Journal of Aquatic Sciences (2010), 5(2): 331-340