Determination of altitude

Ideally, the altitude of a sea-level indicator is determined initially in relation to a nearby temporary bench mark (TBM) and, thereafter, via surveyed bench marks (subsidiary datum-levels) in relation to a reference datum-level (which in certain cases will be a national reference datum-level). For purposes of correlation between reference datum-levels in areas remote from each other, it may be necessary for the altitude of a datum-level recorded on a TBM located near to a sea-level indicator to be determined in relation to a ‘local tidal datum-level’. Examples of local tidal datum- levels that have been used are: high water mark of ordinary spring tide at a chosen site or sites on a chosen day or days; high water mark during calm conditions, as indicated by the upper limit of drifted Enteromorpha spp.; mean level of lower low water spring tide; the level of the lowermost reach of a river possessing a gentle gradient; the upper limit of Fucus vesiculosus; the upper limit of barnacle growth. Tide tables, giving predicted levels of the sea at sites devoid of permanent bench marks, also may be used in correlation between one area and another; problems encountered in Kenya, together with possible solutions, provide examples. Comparison of several national reference datum-levels, eg. in NW Europe, may be made by use of a correlation table. Measurement of the vertical distance (altitude) above or below a reference datum-level should be made by surveyor’s level and staff. Altitude determination at offshore sites presents special problems; a standard procedure to resolve these problems is recommended, but practical issues may necessitate the use of a simplified (and less reliable) procedure.