Transcript Projection Systems for Ireland

Pole
Pole
Z axis
P
Lat
Y axis
Long
X axis
New Map Projections for Ireland
Ken Stewart
True Origin
Northings
False Origin
Eastings
The current situation
• The Ireland 1965 geodetic datum and
1975 adjustment
• The Irish Grid
• IRENET95 – the realisation of
ETRF89
• Transformations
– Level 1 - block shift (~2m)
– Level 2 - 7 Parameter Helmert (~1m)
– Level 3 - 3rd Order Polynomial (~0.4m)
A GPS projection for Ireland
•
•
•
•
Why a GPS compatible mapping system?
Proposed projection system and parameters
Effects of changing projection and grid
Implementation choices and issues
Zero Order Network
Z001
Key to GPS Stations
IRENET
GB
Fiducial (IGS)
Z002
Z003
Z004
Z005
Z012
Z006
Z007
Z008
Z009
Z010
Z011
Z097
Z096
Z095
Z099
HERS
ZIMM
MADR
ONSA
KOSG
WETT
Passive GPS Network
Z001
D004
D002
IRENET95
D001
• 172 stations throughout
Ireland
D028
D027
D059
D060
D043
D046
D049
D047
D058
D048
D061
Z004
D051
D063
D065
D064
D069
D077
D084
D097
D157
D132
D096
D095
D103
D104
D118
Z010
D130
D123
D102
D105
D117 D129
D115
D116
D110
D107
D106
D114
D108
D109
D111D113
D112
D128
D160
D124
D161
D163
D162
D164
D121
D122 D126
D099
D101
D158
D159
D120
D119
D098
D100
Z009
D127
D082
D089
D093
D139
D143
D131
D092
D094
D140
D137
D144
D145
D142
D146
D148
D173
D147
D149
Z007 D151D150
D152
D153
D154
D155
D156
D088
D087
Z008
accuracy:
Z006
D086
D085
D091
D090
D141
D081
D080
D079
D083
D136
D138
D070
D076
D074 D075
10mm in plan
15mm in height
D071
D073
D072
• ETRF89 coordinates
D133
D135
Z005
D068
D067
D078
D055
D134
D066
• 42 sites in NI
D022
D050
D062
D020
D021
D056 D037
D035D036
D039
D038
D034
D040
D042
D054
D041
D052 D053
D033
Z003 D044
D045
Z002
D019
D023
D024
D032
D029
D030
D018
D025
D031
D007
D008
D010
D017
D016
D015
D026
D014
D013
D009
D011
D012
D006
D005
D003
D166
D165
Z011
D169
D168
D172
D171
D125
D167
D170
Active GPS Network
16 Active GPS sites in Ireland
• OSi (12)
Athlone
Kilkenny
Galway
Cork
Limerick
Sligo
Dublin x 4
• OSNI (3)
Omagh
Kilrea
Belfast
• Commissioners of Irish Lights (3)
Tory
Loop
Mizen
• ETRF89 coordinates, accuracy of
10mm
Why change?
•
•
•
•
Accuracy
User pressure
International comparability
Shortcomings of current projection
Proposed Projection: Transverse Mercator
• The north-south dimension is greater than
the east-west.
• Conformal / orthomorphic, and therefore:
– The relative local angles about a point on the map are
shown correctly.
– Although a large area must still be shown distorted in
shape, its small features are shaped essentially correct.
– The local scale in every direction around any one point
is constant.
Principles
•
•
•
•
•
Backward compatibility
Precision
Wide availability
Global fit
Meeting all users’ needs
Components - a reminder
• A mapping system comprises of three
elements:
– Reference Ellipsoid
• GRS80
– Geodetic Co-ordinate Reference Frame
• ETRF89 (IRENET95)
– Associated projection
• Three projections have been identified
Which TM?
• The aim of the new map projection
– to realise the accuracy of the European
coordinate reference frame ETRF89
(IRNET95), while minimising distortions
within the new mapping system and
differentiating it from the current system (Irish
Grid).
Existing TM Projection
• Classically designed TM projection.
• Scale on the central meridian of 1.000 035
• Unsuitable because:
– Does not minimise the effects of projection
distortions
– Not an international standard
– Confusion with co-ords, 55 metres difference
from Irish Grid
UTM
•
•
•
•
International Standard
Ireland is in zone 29
CM is 9 degrees West of Greenwich
Parts of Antrim & Down outside nominal
zone width
• Adoption would result in an extra digit (7)
in the northing co-ordinates compared to
Irish Grid
Irish Transverse Mercator
• A new classically designed TM projection
• True origin and CM defined in the IG will be
maintained to distribute evenly the distortions of
projection
• Scale factor of 0.999 82 on the CM
• Two standard parallels
• Minimises the effects of scale change
• Allows different false origin and non-similar
coordiantes
ITM Grid Coordinates
• Reposition false origin
• Provides significantly
different grid coordiantes
• No additional mapping
distortions
Inner co-ordinates: Irish Grid
Outer co-ordinates: ITM
Proposed Projection Parameters
Current
Reference Ellipsoid
Central Meridian
Scale on CM
True Origin
False Origin
ø
λ
Proposed
IG
UTM
ITM
Airy (modified)
GRS 80
GRS80
8° West
9° West
8° West
1.000 035
0.999 600
0.999 820
53° 30' North
8° West
0° North
9° West
53° 30' North
8° West
200 000 W
250 000 S
500 000 West
0 South
600 000 West
750 000 South
Effects of changing Projection
•
•
•
•
Distance
Area
Rotation (UTM projection)
Therefore, mapsheets and sheet edge
matching
– If map sheets remain the same size, they will contain different data
– This is a good time to pull away from mapsheets and develop tools
for managing the data collection, maintenance and supply of a
seamless database
Current
IG
Proposed
UTM
Airy Modified
Projection
effects on
scale factor
ITM
GRS80
Maximum scale correction:
On West Coast
1. Over 100 m
2. Over 1 km
1.000 390
0.999 723
1.000 175
+ 3.9 cm
+ 39.0 cm
- 2.8 cm
- 27.7 cm
+ 1.8 cm
+ 17.5 cm
Plotable accuracy @ 1:1,000
3. Max. Distance
20 cm
0.5 km
20 cm
0.7 km
20 cm
1.2 km
Minimum scale correction:
On central meridian
4. Over 100 m
5. Over 1 km
1.000 035
0.999 600
0.999 820
+ 0.4 cm
+ 3.5 cm
- 4.0 cm
- 40.0 cm
- 1.8 cm
- 18.0 cm
Plotable accuracy @ 1:1,000
6. Max. Distance
20 cm
5.7 km
20 cm
0.5 km
20 cm
1.1 km
Maximum scale correction:
On East Coast
1. Over 100 m
2. Over 1 km
1.000 377
1.000 259
1.000 162
+ 3.8 cm
+ 37.7 cm
+ 2.6 cm
+ 25.9 cm
+ 1.6 cm
+ 16.2 cm
Plotable accuracy @ 1:1,000
1. Max. Distance
20 cm
0.5 km
20 cm
0.8 km
20 cm
1.2 km
Variation of scale factor at 53.5 deg N
1.000400
1.000000
Range of scale correction:
355 ppm
659 ppm
355 ppm
0.999800
Number of standard parallels
in Ireland (where scale factor
= zero):
0
1
2
314 268 E (IG)
321 269 E (IG)
078 739 E (IG)
0.999600
11
10
9
8
7
6
L o ng it ud e West ( d eg rees)
IG
ITM
UTM
5
Scale Factor
1.000200
Comparison of effects on area
Current
IG
Proposed
UTM
Airy Modified
Area (1 hectare):
10,000.0 m 2
Area difference from IG
Plotable accuracy @ 1:1,000
1. Best area measurement
2. Achievable accuracy
Comments
ITM
GRS80
9989.7 m 2
9998.3 m 2
- 10.3 m 2
- 1.7 m 2
Within tolerance
Within tolerance
20 cm
9960.0 m 2
± 40.0 m 2
Maintaining the IG parameters, but substituting the GRS80 ellipsoid gives an area of 10002.4
m 2, i.e. each hectare measures 2.4 m 2 larger.
Summary of Projection Effects
• The effect of scale change on distance:
– The UTM map projection has both the largest scale correction (- 40 cm
over 1 km on the central meridian) and largest range of scale correction
(659 ppm).
– The ITM map projection best minimises the effects of scale change, with a
maximum scale correction of –18 cm over 1 km on the central meridian.
• The effect of scale change on areas:
– In terms of plottable accuracy associated with the two NMAs’ large scale
mapping products, it can be considered as insignificant in the adoption of
either UTM or ITM map projections to be associated with the ETRS89
coordinate reference system and the GRS80 reference ellipsoid.
• UTM map projection would require additional software development
due its rotation effect on transformed IG mapping (~ 0.8 degree).
Conclusions
•
•
•
•
•
The current situation is not ideal
Changing is not painless
Overall, change is needed
User involvement is essential
Systems need to fit the principles
Projection adopted
Agreed to offer users choice in projection, but advise users as
follows:
• Those in Land and Engineering Surveying requiring precision
measurement in a plane co-ordinate system require minimal distortion,
scale correction and area differences. This is clearly provided by ITM.
• Those in GIS and working in the international and European context
should use UTM, as this provides the most standardised international
way in which co-ordinates can be expressed in 2D.
• Traditional map users, already used to IG, can continue to use IG and
we will ensure backward compatibility.
Remaining issues
•
•
•
•
•
•
Timetable
Consultation and information
On-the-fly transformations?
Product design
Small-scale products
Related activities: Real Time GPS Network