What is the difference between lat and ahd

However the water does not adjust itself immediately to a change of pressure but it responds to the average change in pressure over a considerable area. The effect of wind on sea level, and therefore on tidal heights and times, is variable and depends on the topography of the area in question.

In general, it can be said that wind will raise the sea level in the direction towards which it is blowing. A strong wind blowing straight onshore will cause the water to 'pile up' resulting in high waters to be higher than predicted. Mean sea level for was assigned a value of 0.

The resulting datum surface, with minor modifications in two metropolitan areas, has been termed the Australian Height Datum and was adopted by the National Mapping Council at its twenty-ninth meeting in May as the datum to which all vertical control for mapping is to be referred. The datum surface is that which passes through mean sea level at the 30 tide gauges and through points at zero AHD height vertically below the other basic junction points.

The levelling network in Tasmania was adjusted on 17 October to re-establish heights on the Australian Height Datum Tasmania. For example, assume the corner of a mining lease in Australia was defined using WGS84 coordinates. If the date of the survey is not known and could be anytime over the past 10 years, there is an uncertainty in the location of the corner of 7cm per year or up to 0. This means the coordinates in Australia are projected forward to the date of 1 January Since then:.

These refinements to the reference frame and many of the local scale distortions had not been reflected in changes to the Australian datum since GDA GDA provides a more robust and accurate datum which is more closely aligned to global positioning systems like GNSS and will ensure that Australian industry, the research community and the public can accurately align themselves and their data.

This re-adjustment was not adopted by all States resulting in both versions being in use prior to adoption of GDA Users need to be very careful, as there remains a lot of data and information based on the old system and there is approximately metres between the two. The AGD datum was designed to fit the shape of the Earth in the Australian region as closely as possible. Further, it was fixed in relation to one central point, Johnston Origin , and therefore was not affected by tectonic plate movement; the datum went with the plate.

The major drawback of this datum was that it could not be rigorously related to international systems such as GPS. The National Transformation Grid overcomes this problem to a very large extent by developing local parameters based on a national grid.

For this reason, the National Transformation Grid is recommended for such transformations. As we have pointed out, this is, strictly speaking, insufficient for a datum. The version of WGS84 is commonly not quoted nor is any reference epoch. For the purposes of charting and navigation however these considerations are generally insignificant.

ITRF at Projections and datum should not be confused. See the explanation about how these differ in Datums - The Basics. Heights are expressed in relation to a height datum. The Geoid To understand height datum it is preferable to first understand the concept of the geoid. The Geoid is an imaginary surface which coincides closely with mean sea-level over the ocean and its extension under the continents.

Because gravity is variable, depending upon things such as variable Earth mass, the geoid is an irregular surface.