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Establishing GPS Control Stations

for Land Boundary Survey

Mr. Simon C W KWOK

Chair of the Geomatics Professional Group,

Royal Institution of Chartered Surveyors Hong Kong (2005-2009)

Chairman of the Land Surveying Division

Hong Kong Institute of Surveyors (2009-2010)

Chief Land Surveyor, Lands Department

9 March 2011

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Land Survey Ordinance (Cap 473)

Section 28

Responsibility of an authorized land surveyor

(1) An authorized land surveyor shall ensure that every

land boundary survey or any part thereof undertaken byhim is carried out, whether by himself or by other persons

under his supervision or direction, in accordance with the

requirements of any code of practice approved under this

ordinance.

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(2) An authorized land surveyor shall ensure that any

field note, survey record plan or land boundary plan

prepared by him or by other persons under his

supervision or direction complies with the requirements of

any code of practice approved under this ordinance, .

(4) An authorized land surveyor shall be personallyresponsible for the accuracy and completeness of every

land boundary plan signed and certified by him .., and

he shall be liable for any loss or damage suffered by any

person as a result of any inaccuracy or incompleteness of

any land boundary plan so signed and certified.

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Code of Practice, 4th Edition (Revised) December 2006

--- approved under Land Survey Ordinance

Part F Establishment of Control Stations using GPS

Appendix F -- Specifications and practice guide for establishingGPS control stations for land boundary survey (October 2004,

version 2.0).

30. Control stations established by using GPS shall comply with the

accuracy standard and quality requirements as stipulated in Part I of

Appendix F.

Authorized Land Surveyors are required to follow therecommended practice for establishing such control stations and

maintaining survey records for quality checking as set out in Part II

of Appendix F.

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The Code of Practice (4th Edition 2006)

Part I The mandatory requirements in accuracy standardand quality requirement

(1) The standard error of any GPS baseline measurement shall not

exceed,

where

=[ 102 +(3L)2 ] mm, (note: L = length of baseline in km)or

= 30 mm;

whichever is the less.

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(2) Control Origin

A new station to be fixed by GPS technique shall be connected to at least

two Hong Kong GPS Control Stations established by the Geodetic

Survey Section of Lands Department and situated within 20 km from the

new station.

A GPS control station shall be checked before it is adopted as a control

origin. The discrepancy between any measured baseline component and

the respective computed baseline component derived from published

values shall be less than 2.

i.e. dLat obs dLat comp < 2 and

dLong obsdLong comp < 2

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(3) Station connection

Every new GPS station shall be fixed by at least 3 independent GPS

baselines connected directly to, at least, two GPS control stations.

(4) Observation sessions

At least two sessions of observation shall be made for each GPS

baseline. The time lapse between observation sessions shall not be

less than 30 minutes at different times of day.

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(5) GPS Baseline processing

All GPS baseline solutions shall be integer ambiguity fixed solutions.

The discrepancy of any baseline component of repeated baselines

(i.e. dLat maximum dLat minimum,

dLong maximum dLong minimum or equivalent)

shall be less than 2 (2)

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(6) Least squares network adjustment

All GPS baselines used for computing the solution shall be

independent baselines.

Every new GPS station shall be fixed by at least 3 independent

baselines obtained from not less than 2 observation sessions.

The residuals of GPS baseline components(i.e. VLat, VLong or equivalent) shall be less than 2.

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A case showing :

how to carry out GPS control survey

how to demonstrate compliance of the requirements of the Code of

Practice.

Purpose of the Job:

provide control points for re-establishment of the boundary of a lot.

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Control Diagram

Site

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Control Diagram

Site

ToNail2

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Use Static GPS Survey to establish 3 GPS control stations.

Accuracy standard: [102 + (3L)2 ] mm or 30mm, whichever

the less, where L = length of baseline in km

Break down 3 traverse stations

Linear misclosure = (10 + 2S / 15) mm, where S = total length

of traverse in metre

Angular misclosure = 30n seconds, where n = number of

survey station in the traverse

Accuracy Standard

Code of Practice of Land Survey Ordinance, 4th edition 2006

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Environment of GPS Control StationNail 1

Nail 1

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Obstruction

Diagram of

Nail 1

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Nail 2

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Obstruction

Diagram of

Nail 2

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Nail 3

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Obstruction

Diagram of

Nail 3

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GPS Control Working Diagram

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Equipment Used

1. Static GPS measurement

One GPS receiver

2. Traversing

One total station

Two prisms

Three tripods

3. Radiation One mini prism

One steel tape

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GPS Measurement Sequence

First Observation Session

Step 1: Occupy Station 1 (15 minutes)

Step 2: Occupy Station 2 (15 minutes)Step 3: Occupy Station 3 (15 minutes)

Second Observation Session




Total time taken for GPS measurement : 1.5 hours

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Time Taken for

Field Measurement

GPS measurements were taken in parallel with the traversing

and shot measurement operation

2 hours measurement time were spent for :

Establish 3 GPS control station

3 traverse stations

Taking shots of the house and other boundary features

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Quality Checking Results : Check Origin

Reference stations: HKST.1, HKWS

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Quality Checking Results

Repeated Baselines Nail 1

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Residual of Network Adjustment Nail 1

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Residual of Network Adjustment Nail 2

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Independent Checking of GPS measurement

by Terrestrial Measurement

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Misclosure of the traverse broken down

from the GPS control points

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