Geodetic Flashcards
What is a semi-dynamic datum?
- Accounts for the ongoing earth deformation in New Zealand.
- Coordinates are published in terms of their values at the reference epoch of 1 January 2000.
What is a fully-dynamic datum?
Coordinates change in a consistent and organised way over time.
What is a datum?
Reference system for describing positions on the curved surface of the earth. (Latitude and Longitude - WGS84)
What is a projection?
Used to convert the curved surface of the earth to a flat surface. (mN and mE - NZGD2000)
What is the difference between a normal-orthopmetric heights and ellipsoidal heights?
- Normal-orthometric heights refer to heights measured above a surface that approximates the mean level of the seas.
- Includes NZVD16 and 13 local vertical datums.
- Ellipsoidal heights refer to ellipsoid (approximate of the earths surface).
- GNSS receivers measure ellipsoidal heights.
Heights can be converted between normal-orthometric and ellipsoidal systems by using a geoid model.
Discussion points
- Network geometry for post-processed surveys.
- Why people enable all baselines instead of following best practice - LINZ guide similar to Queensland Department of Transport and Main Roads.
- Is GNSS planning still relevant when there are so many satellites available?
- Try to undertsand why EL2T and BX87 didn’t fit very well with the rest of the geodetic marks used.
What is a minimally constrained network adjustment?
Checking the observations in terms of themselves without influence from the wider network. Can indicate erroneous data. Minimally refers to constraining the minimum number of marks (i.e. 1) to be able to get a solution.
What is a constrained network adjustment?
Checks the observations in terms of the wider network. Multiple marks are constrained and any error in these marks gets forced into the network.
Standard error of unit weight - what is it and when is it important to be close to 1?
Standard error of unit weight indicates how close the estimated error values (a-priori) are to the observed value. An SEUW of 1 means that they are the same. Important when using different types of equipment (i.e. TS & GPS) or different session lengths are being observed.
What is NZGD2000 and how do you explain it? (what is it used for, when was it introduced, type of datum)
- NZGD2000 is the coordinate system for surveying, mapping, and positioning in New Zealand.
- It was introduced in 1998 to replace the New Zealand Geodetic Datum 1949 (NZGD1949).
- NZGD2000 is a geocentric datum that is based on a global framework of reference stations, the International Terrestrial Reference Frame (ITRF).
What is a local circuit and how come there are so many of them?
- NZGD2000 circuit for an area.
- Transverse mercator projection.
- Many circuits to reduce distortion.
What is NZTM and what do you need to be careful of when using it?
- Transverse mercator projection.
- Based on NZGD2000.
- Exhibits a low level of disortion at the east-west extents.
- Scale factor - grid distances vs ground distances.
How was the deformation model applied to your project?
- Deformation was not applied as TBC didn’t have the ability to use a deformation model at the time the observations were processed.
- Wasn’t considered to be an issue due to the small/local area that the survey was undertaken (i.e. has assumed that there is no distortion over this area).
- This could have been an issue if the survey was undertaken at a larger scale or if coordinates needed to be consistent with distant points.
What is a deformation model? (3)
- Models the movement and distortion of NZ caused by the continuous movement of tectonic plates and deformation from significant earthquakes.
- It is a time dependent transformation, which means you need to know not only the coordinates of the points being converted, but also the time at which the conversion applies.
- Model includes both a secular velocity model and “patches” which account for specific deformation events (e.g. earthquake).
Why did New Zealand change to NZGD2000?
- Before NZGD2000 was adopted surveying and mapping in New Zealand was in terms of NZGD1949.
- This datum was not accurate enough for modern survey work.
- Its initial accuracy was limited by the capabilities of the survey equipment available between 1900-1940’s.
- Ongoing earth deformation that has moved points up to 2.5m since its definition.
- It is also inefficient to work with NZGD1949 coordinates in an international environment where positioning, navigation and information systems relate to global Earth models.
