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Rabu, 31 Disember 2008

GPS WORKS

The actual principle of GPS is very easy to appreciate, since it is exactly the same as traditional “triangulation” (although this is not quite correct, as GPS does not use angles). If one imagines an orienteer needing to locate them on a map, they first need to be able to find at least three points that they recognize in the real world, which allows them to pinpoint their location on the map. They can then measure, using a compass, the azimuth that would be needed to take them from the point on the map to their current position. A line is then drawn from each of the three points, and where the three lines meet is where they are on the map.

Translating this into the GPS world, we can replace the known points with satellites, and the azimuth with time taken for a signal to travel from each of the known points to the GPS receiver. This enables the system to work out roughly where it is located - it is where the circles representing the distance from the satellite, calculated on the basis of the travel time of the signal, intersect. Of course, this requires that the GPS locator has the same coordinated time as the satellites, which have atomic clocks on board. To do this, it cross-checks the intersection of the three circles with a fourth circle, which it acquires from another satellite.

If the four circles no longer intersect at the same point, then the GPS system knows that there is an error in it’s clock, and can adjust it by finding one common value (one second, half a second and so on) that can be applied to the three initial signals which would cause the circles to intersect in the same place. Behind the scenes, there are also many complex calculations taking place which enable the system to compensate for atmospheric distortion of the signals, and so forth, but the principle remains the same.

GLOBAL POSITIONING SYSTEM (GPS)

DEFINITION

Originally conceived as a navigation aid for the military, the Global Positioning System, or GPS, has since grown from relatively humble beginnings as different supporting technologies have been developed, some off which are within reach of consumer budgets. All that GPS does is provide a set of coordinates which represent the location of the GPS unit with respect to its latitude, longitude and elevation on planet Earth. It also provides time, which is as accurate as that given by an atomic clock. The actual application of the GPS technology is what leads to such things as navigation systems, GPS tracking devices, GPS surveying and GPS mapping. GPS in itself does not provide any functionality beyond being able to receive satellite signals and calculate position information. But it does that very well

DIFFERENTIAL GLOBAL POSITIONING SYSTEM (DGPS)

Differential GPS is an enhancement and a method of increasing the accuracy of positions derived from GPS receivers. There are 3 main components in performing this system which are;

Ä Base station.
Ä Rover station.
Ä Radio link.

It uses a network of fixed ground based reference station to broadcast the difference between the positions indicated by the satellite system and the known fixed position. This station broadcast the difference between the measured satellite pseudo range and actual (internally computed) pseudo range, and receiver station may correct their pseudo range by the same amount. This correction will be sent by radio link.With DGPS, position accuracy is improved, going from 30 metres to better than 10 metres. This system requires at least 2 GPS instruments (receivers & antennas) transmitting positions from at least 4 GPS satellites. 1 GPS instrument set up on a base station with known position and the other 1 set up on a static or dynamic rover station with unknown position.

POSITIONING SATELLITES

GPS receivers use the principle of "ranging". The receiver measures the distance from a location on earth to the positions of several satellites to determine the latitude and longitude of the position on earth. In all GPS observation, at least 4 GPS satellites need to be observed simultaneously.


· 1 satellite:
Suppose a GPS receiver turned on and picks up the signal of 1 satellite. The receiver tells the location of the satellite and the distance between the receiver and the satellite. But this single measurement is not a big help. At best, we can conclude that the point position given is within a rather large circle whose circumference is determined by the distance from the satellite. We are still lost.
· 2 satellites.
Now suppose that the GPS receiver picks up signals from another satellite. This helps to narrow down the location and the location is somewhere within the intersection of 2 large circles. We still do not know the exact position of the point, but things are starting to look better.

· 3 satellites.
Let’s go one better. If the receiver picks up signals from a third satellite, then the position will be known to be within the intersection of 3 circles. Now we have the position but no check on its quality.
· 4 satellites.
With 4 satellites we will get a precise point for the position and also the elevation. Here, we have a position that is accurate to 30 metres.

Land Survey Datum (LSD)

= It is used for engineering project.
= Usually used the vertical datum from established BM.
LSD is differing from MSL because reduced level from established BM is referring to MSL at Klang Port.
=Reduced sounding refer to LSD can merge with land details refer to vertical datum from established BM.

Chart Datum (CD)




=This is the datum plane adopted by the national authority for its published chart and tidal predictions.
= The datum only is used around of tidal observation.
= It is different at different locations.
= Depends on the range of tidal or water level.
= It will only be the same as the SD when the Hydrographic Office has analyzed the survey data and confirmed the suitability of field surveyor’s choice. The surveyor cannot establish CD in field.
= CD is defined by the International Hydrographic Bureau (IHB) as a level below which the tide will but seldom fall.
= In none-tidal waters, CD is normally established at MSL.
= Because LAT is so difficult to define precisely, CD for Admiralty and Australian charts has been more broadly defined as the level below which no predicted tide shall fall more than 0.1m.

Sounding Datum (SD)

= This is the datum plane to which soundings are reduced in the course of a hydrographic survey and is therefore the datum used when compiling the Fair Chart or Final Tracing.
= It is the datum which the field surveyor will establish and adopt.
= Ideally SD should be the same as Chart Datum (CD), but may well differ particularly when the original survey is very old.
= One of these methods is to be used when establishing datum;
· Recover a previously established SD or CD in the survey area by leveling from benchmarks.
· Transfer datum from a place close to the survey area where CD or SD is already established, by taking simultaneous tidal observations over a period.
· Establish a new datum from observations.

Lowest Astronomical Tide (LAT)

= LAT is the lowest level that can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions.
= There is no simple formula or rule available to calculate LAT from observations or tidal constituents.
= It can only be obtained properly by studying tidal predictions covering several years (18.6 years ideally), as level of LAT will not be reached every year.

Mean Tide Level (MTL)

=This must never be confused with MSL.
= MTL is defined as the mean of all HWS and LWS over a period of time.
=  It is not (at present) possible to convert simply from MSL to MTL (or vice versa) at any point.
= MTL and MSL will only coincide when tidal curve is a perfect sine curve and this rarely occurs.

DEFINITION OF DATUMS

Mean Sea Level (MSL)

= MSL is the natural reference level of the geoid, and is defined as the average level taken up by the sea along a long period (18.6 years if possible).
= In practice it is obtained from the mean of all tide readings or MSL is the average level taken up by the sea.
= It coincides very nearly with the geoid.
= The accuracy of MSL depends on a long period of tidal observation.
= The best result can be obtained with 18.6 years observation.
= MSL is the best vertical reference level we have and as such is the basic reference level for tides.
= Observed MSL is subject to change due;
· Polar melting or glacial activity.
· Meteorological effects (rain, wind, pressure).
· Short period effects (sea, swell, surges, etc.).
· Tectonics (crustal changes).
· Astronomical effects (changes in MSL due to the combined effect like seasonal meteorological variation).
· Mean of all tide readings over a long period (30 days to 18.6 years).
· Multiple of 25 hourly observations (preferably 3 days to 75 hours).
· Mean of 25 hourly observations (poorer value).

DATUMS

When conducting a survey, the surveyor needs to measure heights and depths above or below certain fixed level, which are referred to as datum’s. The datum to which depths are to be reduced is fundamental to any survey and will contain details of how this is to be established.

Sabtu, 13 Disember 2008

Khamis, 11 Disember 2008

Coordinate Transformation

There are several types coordinate that we use in cadastral survey in world. Nowaday, the cassini coordinate show in the certified plan in cadastral. This coordinate system will exchange to the new coodinate system. That we call GDM2000.. i have a problem about how to convert cassini coordinate to rso coordinate. i hope somebody can solve my problem. I just want to know the formula.that it...


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