Table of Contents
What does the datum tell us?
A datum is a model of the earth that is used in mapping. The datum consists of a series of numbers that define the shape and size of the ellipsoid and it’s orientation in space. A datum is chosen to give the best possible fit to the true shape of the Earth. There are a large number of datums in use.
What are the two types of datum?
There are two main datums in the United States. Horizontal datums measure positions (latitude and longitude) on the surface of the Earth, while vertical datums are used to measure land elevations and water depths.
What marks a datum?
The artisan nominates two straight edges on a workpiece as the “datum edges”, and they are marked accordingly. One convention is to mark the first datum edge with a single slanted line (/) and the second with double lines (//). For most work, the datum references of the workpiece need to be square.
What does tidal datum mean?
A tidal datum is a standard elevation defined by a certain phase of the tide. Tidal datums are used as references to measure local water levels and should not be extended into areas having differing oceanographic characteristics without substantiating measurements.
How do you identify a datum?
Datums are identified by capital alphabetical letters in squares. Datums are identified by attaching these with leader lines to an individual surface, extension from a surface or to a feature control frame.
What is chart datum level?
The chart datum is the level of water that charted depths displayed on nautical charts are measured from. The chart datum is generally a tidal datum; that is, a datum derived from some phase of the tide. Common chart datums are lowest astronomical tide and mean lower low water.
What is the datum for tide heights?
Mean lower low water (MLLW) is the average height of the lowest tide recorded at a tide station each day during a 19-year recording period, known as the National Tidal Datum Epoch as used by the United States’ National Oceanic and Atmospheric Administration.
What is datum dimension?
A datum dimension is a group design object. It allows for the dimensioning of a linear distance of a collection of objects, relative to a single reference object. The first object chosen is the ‘base’. All subsequent objects are relative to this first object.
What is datum chart?
A chart datum is the water level surface serving as origin of depths displayed on a nautical chart. A chart datum is generally derived from some tidal phase, in which case it is also known as a tidal datum. Common chart datums are lowest astronomical tide (LAT) and mean lower low water (MLLW).
What does chart datum means?