<P> Hydrography, mostly for reasons of safety, adopted a number of conventions that have affected its portrayal of the data on nautical charts . For example, hydrographic charts are designed to portray what is safe for navigation, and therefore will usually tend to maintain least depths and occasionally de-emphasize the actual submarine topography that would be portrayed on bathymetric charts . The former are the mariner's tools to avoid accident . The latter are best representations of the actual seabed, as in a topographic map, for scientific and other purposes . Trends in hydrographic practice since c. 2003--2005 have led to a narrowing of this difference, with many more hydrographic offices maintaining "best observed" databases, and then making navigationally "safe" products as required . This has been coupled with a preference for multi-use surveys, so that the same data collected for nautical charting purposes can also be used for bathymetric portrayal . </P> <P> Even though, in places, hydrographic survey data may be collected in sufficient detail to portray bottom topography in some areas, hydrographic charts only show depth information relevant for safe navigation and should not be considered as a product that accurately portrays the actual shape of the bottom . The soundings selected from the raw source depth data for placement on the nautical chart are selected for safe navigation and are biased to show predominately the shallowest depths that relate to safe navigation . For instance, if there is a deep area that cannot be reached because it is surrounded by shallow water, the deep area may not be shown . The color filled areas that show different ranges of shallow water are not the equivalent of contours on a topographic map since they are often drawn seaward of the actual shallowest depth portrayed . A bathymetric chart does show marine topology accurately . Details covering the above limitations can be found in Part 1 of Bowditch's American Practical Navigator . Another concept that affects safe navigation is the sparsity of detailed depth data from high resolution sonar systems . In more remote areas, the only available depth information has been collected with lead lines . This collection method drops a weighted line to the bottom at intervals and records the depth, often from a rowboat or sail boat . There is no data between soundings or between sounding lines to guarantee that there is not a hazard such as a wreck or a coral head waiting there to ruin a sailor's day . Often, the navigation of the collecting boat does not match today's GPS navigational accuracies . The hydrographic chart will use the best data available and will caveat its nature in a caution note or in the legend of the chart . </P> <P> A hydrographic survey is quite different from a bathymetric survey in some important respects, particularly in a bias toward least depths due to the safety requirements of the former and geomorphologic descriptive requirements of the latter . Historically, this could include echosoundings being conducted under settings biased toward least depths, but in modern practice hydrographic surveys typically attempt to best measure the depths observed, with the adjustments for navigational safety being applied after the fact . </P> <P> Hydrography of streams will include information on the stream bed, flows, water quality and surrounding land . Basin or interior hydrography pays special attention to rivers and potable water although if collected data is not for ship navigational uses, and is intended for scientific usage, it is more commonly called hydrology . </P>

What is the difference between hydrographic and bathymetric survey
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