Caribbean Data Management Community Presents:

Caribbean Open Data Management Community

Monthly Webinar Series https://collaboration.worldbank.org/groups/caribbean-open-data-management

Caribbean Data Management Community

Presents: “Datum issues in the integration of old and new geospatial data”

(Tuesday, May 21st, @10am OECS)

MONTHLY WEBINAR With:

Dr. Keith MillerSenior Lecturer at the University of West Indies

[email protected]

To learn more, please visit the Caribbean Data Management Community site:https://collaboration.worldbank.org/groups/caribbean-open-data-management

Contact: Bradley Lyon ([email protected])

mailto:[email protected]

https://collaboration.worldbank.org/groups/caribbean-open-data-management





Presentation plan• Principles - models of the Earth• Uniqueness of traditional datum

definition. • Need for an international datum• Issues associated with moving spatial

data between datumso Horizontalo Vertical

• Support in geospatial data management through UWI



Earth gravity and potentialLevel surface - a surface of constant potential. Level surfaces

are not parallel and do not cross.

Gravitational acceleration – rate of change of potential along a plumb line.

SEA ROCK Medium of different densites

Surfaces of constant potential, which provide level surfaces.

Earth’s terrain



The Geoid as a reference surface

• A surveyor levels an instrument on a physical level surface.

• Latitudes and longitudes measured independently do not provide relative data about two points.

• Level surfaces are not suitable for mapping purposes

Φ Φ

Geoid - a surface of constant potential corresponding to Mean Sea Level



The Spheroid as a reference surfaceSpheroid – an ellipse rotated about the pole that approximates the

geoid.

Different people measured the shape of the Earth in different parts of the World to produce different results.

a b

a - semi-major axis b - semi-minor axis

flattening f = (a-b)/a

eccentricity squared e2 = (a2-b2 )/a2

Name Year a (m) 1/f

Delambre 1800 6375653 334

Laplace 1802 6376614 305

Delambre 1810 (II) 6376428 311.5

Zach 1812 6376385.33 310

Airy 1830 6377563.39 299.32

Everest 1830 6377276.34 300.8

Bessel 1837 6377156.87 300.71

Everest 1847 6376701.34 311.04

Hayford 1906 (II) 6377945 296.5

Heiskanen 1929 (II) 6378391 297



Spheroids of the AmericasThere are over 150 different spheroids defined worldwide. Some of those for the Americas as computed by Clarke are:

Name Year a (m) 1/f

Clarke 1856 (I) 6377929.41

298.07 Variations II and III exist

Clarke 1858 6378293.645

294.26 Used in Trinidad & Tobago

Clarke 1858 (III) 6378558.24

280.4 Variations I and II exist

Clarke 1860 (I) 6378402.46

287.78 Variations II and III exist

Clarke 1880 (II) 6378446.36

290.04 Variations I and III exist

And Internationally

International

1924 6378388 297 Used all global data available

There is some confusion over which of the 27 different definitions of the foot Clarke used.



Defining a datum point

Geoid

Spheroid at datum 2

Spheroid 2

Spheroid at datum 1

Regional Datum point 1

Regional Datum point 2

Greenwich

Vertical

Levels

N E

Perpendicularto spheroid

Tangentto spheroid

P

pole

equator

Greenwichmerid ian

b

a

A point observed on a level surface is made equivalent to coordinates on the spheroid

The spheroid is connected to the level surface at this point, so different datums place the spheroid differently in space.



Importance of metadataThe Provisional South American Datum of 1956 uses the International 1924 spheroid and has La Canoa in Venezuela as its datum point.The coordinates of Naparima Hill, Trinidad on PSAD56 are:1017’02.416” North 6127’22.606” West

The Naparima datum of Trinidad uses the International 1924 spheroid, with Naparima Hill as the datum point.

Observed astronomical coordinates, and hence location of this point on the Naparima datum are:1016’44.860” North 6134’22.620” West

The same point is displaced by some 650 metres, yet both coordinates are correct.

Coordinates must be supported with datum information.



A datum is provided by:• A point in latitude and

longitude.• Definition of a

spheroid.• Orientation by

direction to another point.

• Scale by measuring a distance.

Triangulation extends the framework across the land mass with points being marked.



First order network for Trinidad



Datums of the Caribbean 1Country Datum Spheroid Projection

AnguillaA4 astro 1957 Clarke 1880 British West Indies TMNAD 1927 Clarke 1866 TM

Antigua USNHO astro 1943 Clarke 1880 British West Indies TM Barbuda NAD 1927 Clarke 1866 TM (with UTM grid)

BarbadosChallenger astro 1938 Clarke 1880 Barbados National TMChallenger astro 1938 Clarke 1880 British West Indies TMNAD 1927 Clarke 1866 BWI TM (with UTM grid)

BelizeNAD 1927 Clarke 1866 TM & UTMBritish Honduras 1922 Clarke 1858 TM Colony Coordinates

Cayman Islands

IAGS astro Grand Cayman Clarke 1866 UTM (formerly TM)

LC 5 astro Cayman Brac. Clarke 1866 UTM (formerly TM)LC 5 astro Little Cayman Clarke 1866 UTM (formerly TM)

ColombiaSouth American 1969 S. American 1969 UTMProv. South Am. 1956 International 1924 UTMBogota Observatory International 1924 Colombia TM

Costa Rica NAD 1927 Clarke 1866 Costa Rica Lambert N&SCuba NAD 1927 Clarke 1866 Cuba Lambert North & South




DominicaM12 astro 1945 Clarke 1880 mod British West Indies TM

NAD 1927 Clarke 1866

Dominican Rep. NAD 1927 Clarke 1866 Dominican Republic Lambert

GrenadaGS 80 astro 1953 Clarke 1880 mod British West Indies TM

NAD 1927 Clarke 1866 British West Indies TM

GuadeloupeGuadeloupe International 1924 UTM

NAD 1927 Clarke 1866 Guadeloupe Gauss Laborde

Guatemala NAD 1927 Clarke 1866 Guatemala Lambert N&S

GuyanaProv. S. American 1956 International 1924 UTM

Local International 1924 British Guiana Colony

HaitiNAD 1927 Clarke 1866 TM (with UTM grid)

NAD 1927 Clarke 1866 Haiti Lambert

HondurasNAD 1927 Clarke 1866 TM (with UTM grid)

NAD 1927 Clarke 1866 Honduras Lambert N&S

Jamaica

Fort Charles Flagstaff Clarke 1866 Jamaica Lambert Metre

Fort Charles Flagstaff Clarke 1880 Jamaica Lambert Foot

NAD 1927 Clarke 1866




Martinique International 1924 UTM

MexicoNAD 1927 Clarke 1866 Angular Polyhedric & UTM

Isla Socorro astro Clarke 1866 UTM

Montserrat M36 astro 1958 Clarke 1880 mod British West Indies TM

NetherlandsAntilles

International 1924 TM

International 1924 UTM

NicaraguaNAD 1927 Clarke 1866 Nicaragua Lambert N&S

NAD 1927 Clarke 1866 UTM

PanamaNAD 1927 Clarke 1866 TM

Panama (Colon) Clarke 1866 Panama Lambert & Polyconic

Puerto RicoPuerto Rico 1901 Clarke 1866 TM & UTM

NAD 1927 Clarke 1866 Lambert

St Croix NAD 1927 Clarke 1866 St Croix Lambert

St Kitts &Nevis

K12 astro 1955 Clarke 1880 British West Indies TM

NAD 1927 Clarke 1866 TM



Abbreviations used• NAD 1927 is the North American Datum

1927 which has an origin at Meades Ranch (3913’26.686” North, 9832’30.506” West) and uses the Clarke 1866 spheroid.

• British West Indies TM is the Transverse Mercator projection that was implemented by the Directorate of Overseas Surveys of Great Britain for the West Indies. It has a central meridian of 62, a scale factor at this longitude of 0.9995, a latitude of origin at the equator and a false Eastings of 400km.



Traditional datums are unique

• There is no spatial relationship between datum points. Each was determined independently on a level surface.

• Until recent times a datum could only be transferred between land masses when a line of sight existed.



Need for a seamless World

• Modern satellite positioning systems operate globally.

• GIS and remote sensing provide global data sets and applications that operate seamlessly between land masses.



A Global Spheroid• Gravity data obtained from satellite motion has provided

better definition of the geoid globally.• A global best fitting spheroid to the geoid was adopted for

use with GPS: the World Geodetic System of 1984 (WGS84) with:a = 6378137m, and 1/f = 298.257223563



Transforming between datumsIn the horizontal component, a number of

methods exist:• Affine – used locally on the projection plane,

implemented within GPS processing software, requires observation of local control

• 3 parameter (block shift) – ΔX, ΔY, ΔZ commonly used, assumes that axis of the spheroids adopted within datums are aligned.

• Molodensky – ΔX, ΔY, ΔZ, δa, δf in practice offers no benefit over the 3 parameter, but is implemented in many hand held receivers.

• Bursa-Wolf (7 parameter ) – ΔX, ΔY, ΔZ, rX, rY, rZ, s accommodates rotations and scale.

• Molodensky-Badekas – ΔX, ΔY, ΔZ, rX, rY, rZ, s and an origin. Improves computation of parameters, but will offer the same results as Bursa-Wolf.



Determining parametersExisting control is observed with GPS so that coordinates of points are known in both datums.Point ΔE (mm) ΔN (mm) ΔU (mm)M20 5 2 -21N227 1 5 29S15 11 5 20S19 7 6 48S70 3 8 7T11 5 13 -37

In Barbados, 2 GPS campaigns were undertaken with some points included in both to obtain an indication of accuracy of GPS observations. Note that GPS is more accurate in the horizontal component. Resulting parameters are used only for horizontal transformations.



Co-located pointsPoint data for Barbados as numeric values for 16 control points used to compute parameters offers redundancy.

Resulting parameters are applied to this data for assessment.

0

12

34

5

67

8

-3.5 -3

-2.5 -2

-1.5 -1

-0.5 0

0.5 1

1.5 2

2.5 3

3.5

The histogram shows that some points do not conform with the general trend.



Issues with existing controlControl in the Caribbean was typically

observed in the 1960’s:• In the case of Barbados control point S38 is located

in the Scotland district where localised deformation has occurred.

• In one case on another island, when a point was destroyed it was rebuilt by the government department, about 100 m from its previous location, but new coordinates were never determined.

• In another instance a farmer moved the concrete monument about 2 m with his tractor, because it was in his way.

• Errors in coordinates are common due to transcription.

Observation methods used and changes through time have lead to inconsistencies in existing control. GPS coordinates can be accurate at the sub-cm level.



Accuracy of transformationsFor Barbados, accuracy levels represented as 68%

confidence levels in East and North for parameters (after removal of point S38) are:• 3 parameter and Molodensky: σE = 0.487 m, σN =

0.437 m• 7 parameter and Molodensky-Badekas: σE = 0.117

m, σN = 0.108 mFor Trinidad, horizontal accuracy levels are about:• 3 parameter: 0.5 m• 7 parameter: 0.5 mBut, may be up to 0.8 m in some areas.Integrity of the original network has not been retained so well in Trinidad where a lot more deformation has taken place.



Spatial variation in accuracyFor Barbados it was also established that orientation

of the local datum and WGS84 differ by 12”. As the 3 parameter transformation does not accommodate rotations then accuracy of these parameters degenerates with distance from the centre of the island.Parameters are applicable across the region in which they are derived. e.g. One of the datums in use in Jamaica is NAD27, but transformation parameters used for NAD27 in the USA or elsewhere would not be applicable in Jamaica. Extrapolation offshore may also be an issue. Across large areas such as the USA, no one set of parameters will work, and techniques to vary parameters across the continent are used.



Which way to transform?Transformation parameters are provided in a

particular direction e.g. Barbados transformation parameters WGS84 to Challenger Astro

3 parameter TX = 32.055 m TY = 301.786 m TZ = 419.928 m

Software will also require data in a given direction.Change in direction between given data and software needs requires a change in sign.In instances where there are only 3 parameters available then set rotations and scale to 0.It is normal to transform from a traditional datum to

WGS84.As already indicated, the traditional datum is less accurate, to take good GPS data onto a traditional datum is degrading the quality of the information.



Be careful with rotation signsThere is a standard convention for positive East or X, and North

or Y in translations. But, the convention for rotations is not so well defined.There are 2 conventions for direction of rotation parameters, the International Earth Rotation Service (IERS) has one convention while the International Association of Geodesy (IAG) adopts the reverse.The solution is to change the signs for the rotations, but knowledge of the convention adopted by an application is required.Generally, Europe adopts the IERS convention while Australia and the USA use the IAG.



Third party parameters (NIMA)

Local GeodeticDatum

Region Transformation ParametersX(m) Y(m) Z(m)

Bogota observatory Colombia 307 6 304 5 -318 6

Provisional South American 1956

mean solution -288 17 175 27 -376 27Colombia -282 15 169 15 -371 15Guyana -298 6 159 14 -369 5

Venezuela -295 9 173 14 -371 15USNHO Astro 1943 Antigua -270 25 13 25 62 25

K12 astro 1955 St Kitts & Nevis -7 25 215 25 225 25LC5 astro Cayman Brac. 42 25 124 25 147 25

M36 astro 1958 Montserrat 174 25 359 25 365 25Naparima Trinidad & Tobago -10 15 375 15 165 15

Puerto Rico 1901 Puerto Rico 11 3 72 3 -101 3

Note the inaccuracies. These parameters were computed a long time ago often from minimal co-located points. These values are often used as defaults in hand held receivers.In Trinidad different companies used different control points to compute parameters and lots of variations exist between results.



Third party parameters (EPSG)



Vertical datums

Sea Level

MSL

Land survey datum is typically Mean Sea Level (MSL).Chart datum is typically Lowest Astronomical Tide (LAT).Vertical reference levels were established in former British Caribbean during the 1930’s, when many datum points were also observed.



Bench marksTriangulation points carry heights, and these were mostly observed using trigonometric heighting, which is not accurate.Differential levelling is used to height bench marks. As these were established along communications routes many have been destroyed by development.



Accuracy of vertical datumsSea level observations made in the 1930’s were short term, often 1 or 3 months. • 18.6 years of data is required to cover a full

astronomical tidal cycle.• In the Caribbean there is seasonal variation in sea

level of 0.1 to 0.2 m• Longer observation periods reduce effects of

atmospheric variation.In Trinidad the estimate for MSL has been validated, but sea level has risen about 0.15 m since it was observed, and in some places vertical deformation of the land mass has had a significant impact on levels assigned to bench marks.In Barbados an error in the reference level used for observations means that MSL is about 0.3 m higher than the land survey datum was when it was established.



Heights and GPS

• Water flows on level surfaces and not on the spheroid.

• The geoid is the level surface that corresponds to MSL

• For drainage and water flow we need to retain heighting conventions relative to the geoid, or MSL.

• For heighting with GPS, a correction (N) needs to be applied. Geoidal models are built in to some receivers.

Terrain

Geoid

Spheroid

P

N

Hh

GPS provides latitude, longitude and height relative to the spheroid.

*



Geoidal models (N) – need checking



A way forwardExisting horizontal control lacks integrity and the WGS84 datum is aligned with international trends in geospatial data management. • It is better to transform existing data to WGS84, with

an appreciation of the limitations of parameters.• In cases where use of the traditional datum is a

requirement (e.g. legislation) then keep the WGS84 data for future use.

• Issue and publish a set of transformation parameters that can be used by all, then at least all transformed data will be compatible.

Retain MSL for vertical control. • Check the existing datum against MSL• Validate the geoidal model that is freely available

from EGM2008



The webinar has ended Thanks to everyone for your participation!

A video recording will be shared on the community site

Caribbean Data Management CommunityPresents:

“Datum issues in the integration of old and new geospatial data”(Tuesday, May 21st, @10am OECS)

MONTHLY WEBINAR With:

Dr. Keith MillerSenior Lecturer at the University of West Indies

[email protected]

To learn more, please visit the Caribbean Data Management Community site:https://collaboration.worldbank.org/groups/caribbean-open-data-management

Contact: Bradley Lyon ([email protected])




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Caribbean Data Management Community Presents: