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Gravity Measurement


Related pages






  • Basic gravity measurement feature:
  • Complete gravity measurement feature :

Some important information is associated with the "gravity station" - i.e. the site where the measurement is made. This is modelled as a gravity station feature, which extends the standard CommonFeatures#Station feature:
  • Gravity Station Feature:

Requirements analysis

Here is the table of attributes associated with a Gravity Measurement as recorded in the consolidated database maintained by GA, with the corresponding XMML properties.

Field name Datatype corresponding XMML property
PROJECT NOT NULL NUMBER(6) relatedFeature/Project
OBSNO NOT NULL NUMBER(8) gml:name or gml:id
STATIONNO NOT NULL NUMBER(12) measurementStation/GravityStation@gml:id or /gml:name
STATIONNAME VARCHAR2(30) measurementStation/GravityStation/gml:name
STATIONTYPE NOT NULL VARCHAR2(1) measurementStation/GravityStation/stationType
Reference Station - measurementStation/GravityStation/referenceStation
DLONG NUMBER(9,6) (component of) gml:position or measurementStation/GravityStation/gml:position
DLAT NUMBER(8,6) (component of) gml:position or measurementStation/GravityStation/gml:position
POSUNITS VARCHAR2(3) gml:position@srsName
ACC_LOC NUMBER(6,3) measurementStation/GravityStation/surveyDetails/SurveyProcedure/positionAccuracy
DATUM NOT NULL VARCHAR2(5) gml:position@srsName or measurementStation/GravityStation/surveyDetails/SurveyProcedure/geodeticDatum
SPHEROID NOT NULL VARCHAR2(10) gml:position@srsName or measurementStation/GravityStation/surveyDetails/SurveyProcedure/geodeticDatum
POSMETHOD VARCHAR2(3) measurementStation/GravityStation/surveyDetails/SurveyProcedure/positionMethod
ELEVATION NUMBER(8,3) (component of) gml:position or measurementStation/GravityStation/position
ELEVUNITS VARCHAR2(4) gml:position@srsName
ACC_HT NUMBER(6,3) measurementStation/GravityStation/surveyDetails/SurveyProcedure/elevationAccuracy
ELEVDATUM VARCHAR2(5) position@srsName or measurementStation/GravityStation/surveyDetails/SurveyProcedure/elevationDatum
ELEVMETHOD VARCHAR2(3) measurementStation/GravityStation/surveyDetails/SurveyProcedure/elevationMethod
ELEVTYPE VARCHAR2(2) measurementStation/GravityStation/elevationType
OBSDATE DATE procedureTime
GRAV NUMBER(12,4) resultMeasureExact
GRAVUNITS VARCHAR2(2) resultMeasureExact@uom
ACC_GU NUMBER(5,2) quality or instrumentUsed/GravityMeter/quality
GRAVDATUM VARCHAR2(1) gravityDatum
CALCDATE DATE gravityCalcDate
GRAVMETHOD VARCHAR2(3) instrumentUsed/GravityMeter/gravimeterCode
GMETERID VARCHAR2(10) instrumentUsed/GravityMeter/serialNumber
HEIGHT NUMBER(9,3) meterHeightOffset
HEIGHTERROR NUMBER(6,3) meterHeightError
HEIGHTMETHOD VARCHAR2(3) meterHeightMethod
TC NUMBER(5,2) terrainCorrection
TCERROR NUMBER(4,2) terrainCorrectionError
TCMETHOD VARCHAR2(3) terrainCorrectionMethod
TCDENSITY NUMBER(4,3) terrainCorrectionDensity
RELIAB NUMBER(1) reliability
ENTEREDBY NOT NULL VARCHAR2(8) recordStatus/RecordEvent/responsible
ENTRYDATE NOT NULL DATE recordStatus/RecordEvent/eventTime
UPDATEDBY VARCHAR2(8) recordStatus/RecordEvent/responsible
LASTUPDATE DATE recordStatus/RecordEvent/eventTime
PARENT NUMBER(8) relatedFeature/Observation

-- TimMackey - 10 Oct 2002 -- SimonCox - 21 Jun 2004

The "Basic" measurement essentially reduces these to the following:

Field name Datatype corresponding XMML property
PROJECT NOT NULL NUMBER(6) relatedFeature/Project
OBSNO NOT NULL NUMBER(8) gml:name or gml:id
STATION   measurementStation/GravityStation (etc)
DLONG NUMBER(9,6) (component of) gml:position or measurementStation/GravityStation/gml:position
DLAT NUMBER(8,6) (component of) gml:position or measurementStation/GravityStation/gml:position
ELEVATION NUMBER(8,3) (component of) gml:position or measurementStation/GravityStation/gml:position
OBSDATE DATE procedureTime
GRAV NUMBER(12,4) resultMeasureExact
GRAVUNITS VARCHAR2(2) resultMeasureExact@uom
GRAVMETHOD VARCHAR2(3) instrumentUsed/GravityMeter/gravimeterCode
GMETERID VARCHAR2(10) instrumentUsed/GravityMeter/serialNumber

This design and selection was based on the following commentary from BobRichardson and TimMackey:

If we start with a model for the principal facts that can be easily used by contractors and the surveys we would need to have at least

Surveyno, Stationno, Latitude, Longitude, Elevation, Obsgrav (and possibly Terraincorr)

in the sample class and

Coordframe, Georeference, Elevunits, Obsgunits and Obsgdatum

in a dataset (not necessarily a survey) class.

Other desirable (but not mandatory) items in the basic sample class include:

Stationtype, Obsgmeterno, Obsgrefstn, Restriction, Comments

Other desirable (but not mandatory) items in the basic dataset class include:

Posunits, TCdensity, TCunits, TCmethod

For datasets lodged by contractors Surveyno is probably common for all samples in a dataset. For datasets being exchanged or distributed by the surveys Surveyno may vary for different samples within the dataset. You will have noticed that Stationtype is still present as it may be used as a reference for other procedures in gravity surveying.

-- BobRichardson, Mineral Resources Tasmania - 24 Oct 2001

Gravity Station

Early in the analysis there was a discussion about the notion of "Station".

You are right about station being unique to gravity. This is because after a number of well defined corrections have been made, the absolute gravity value at a station is time invariant (although its accuracy depends on the observation method). Many stations are just samples but there are also special stations which are used as reference points for other observations elsewhere (usually made with relative instruments) or which are used for checking purposes within a survey. Common station types are:

  • Absolute measurement - a station whose absolute gravity value is determined directly with high accuracy (a loose term) either to be used as a reference for other surveys or because the location is such that a good quality relative measurement could not be made.

  • Base station - this is not simple as there are different types of base stations. AGSO have established a network of "primary" tie (base or isogal) staions around Australia which are the primary gravity reference for all other measurements made. State surveys, contractors etc establish other base (or tie) stations which are referenced to these and are of varying accuracy. The family of base stations provide absolute value references and may also be used for drift and tidal corrections during gravity surveys.

  • In my view a control station is a particular class of base station - this may need to be checked with AGSO and others.

  • The detailed survey or traverse station really relates to an observation - a station may be read as part of a detailed survey and also as part of a conventional survey.

  • Ground transport survey and helicopter transport survey do not necessarily relate to an entire survey - they are access methods to a sample point. There may be more than one method within a survey.

I think station type is still important. A typical survey may start by establishing a network of base/control station that are linked to tie or isogal stations. These are then used as references for the rest of the survey which may include double read stations to help with loop adjustments. The physical marking of station locations is also commonly a function of station type with markers varying from brass plaques to a piece of flagging tape or even none at all. Preparations for a new survey include searching for existing isogal, tie and base stations within the survey area.

I don't know if you have seen the AGSO best practice in gravity manual. If not it may help a little although some of the processing is not always used by contractors etc. The manual is at http://www.agso.gov.au/geophysics/gravimetry/codepage.html

-- BobRichardson, Mineral Resources Tasmania - 22 Oct 2001

A station from a previous survey may be used as a base station for a new survey but it retains its old station number. It is only used to tie the new survey to the regional network and ffrom the principal facts point of view wouldnt be classed as part of the new survey. Of course it would be included in a set of raw data.

-- AlanWillocks, Victoria - 22 Oct 2001

OK - so there is a genuine suite of station types:

  1. Absolute - something measured in some very special way and used as an absolute value to tie in to (maybe) global datasets?
  2. Base station - is this continuously monitored? Will it normally have an Absolute measurement associated with it?
  3. Control station - this is some intermediate status? Bob thinks this is just another kind of Base station? Is this a real type??
  4. detailed or traverse stations - doubly observed for levelling corrections as part of a survey.
  5. ordinary stations, visited either by helicopter or truck!

I think I understand 4 & 5 now, and maybe 1, but it sounds like there is some ambiguity in differentiating 2 & 3 - in some surveys a base station will be part of the Fundamental Gravity Network (in which case they may have an Absolute value?) while in others they will merely be convenient temporary base stations set up for this survey.

It looks to me like there may be two distinct functions here.

On the one hand, "Absolute" measurements have that status independent of the current survey of interest. It is a genuine absolute quality flag.

On the other, "base station", "traverse station (tie point)" and "ordinary" station are a functional status for processing a /survey/, and the same station may play different roles in different surveys. Maybe what is really needed is a "role-list" attached to the /survey/ which lists (e.g.) the base stations, the tie points, and the ordinary readings /for this survey/, to be used in the context of interpreting the /survey/.

Meanwhile, what is a "Control Station" and where does this fit in to a survey or station perspective?

-- SimonCox - 22 Oct 2001

WFS demonstrator

A prototype WFS demonstrator was built by Annie Boyd as follows:

Debug interface to WFS serving gravity measurements

%ACTION{ due="21-Mar-2003" creator="Main.SimonCox" uid="000003" state="open" created="17-Mar-2004" who="Trash.XMMLSponsorGGIPAC" }% Bob Richardson and Alan Willocks to critique the encoding for Gravity Measurements %ACTION{ due="21-Mar-2003" creator="Main.SimonCox" uid="000004" state="open" created="17-Mar-2004" who="Main.TimMackey" }% Critique the encoding for Gravity Measurements
Topic revision: r10 - 15 Oct 2010, UnknownUser

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