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OLLIVIER & CO New Zealand Projections
Projections Workshop 2004
• Basic Projection Steps
• NZ Transverse Mercator
• Projections in ArcGIS 9.0
• Reprojecting Images
• Discussion
OLLIVIER & CO New Zealand Projections
Moving between Projections
Projection: NZMG
Units:metres
Projection: Geographic
Datum: Geodetic 1949
Units: lat/long degrees
Projection: NZTM
Units:metres
Projection: Geographic
Datum: NZGD2000
Units: lat/long degrees
Datum Transformation3 or 7 parameter equationor Distortion Grid (NTv2)or None if same datum
Unproject
Project
Unproject
Project
The key is getting the datum transform working
OLLIVIER & CO New Zealand Projections
Survey Local Circuits
• They are all Transverse Mercator projections• A different series for each datum (1949 and 2000)• A different origin for each circuit (28 trig points)• A different scale factor for some (Mt Eden and Taieri)• Different false origins for new series (for identification)• There is no easy or exact conversion 1949 <-> 2000• NZMG and 1949 circuits are same datum [GEO 1949]• NZTM and 2000 circuits are same datum [NZGD2000]• GPS coordinates are in WGS84 ~ NZGD2000
OLLIVIER & CO New Zealand Projections
NZTopo50 - New Sheet Layouts
New Zealand Transverse Mercator
Metric Topographic Sheets
1:500 - 1:50,000 series
Useful for NZTM Aerial photo tilesNot promoted by LINZ yet
but coming….
OLLIVIER & CO New Zealand Projections
Recognising the projection or datum
Distinguish lat/long datum coords by 200 m offset North East (10,190)
Distinguish projected coordinates by different false origins and ranges!
X is always less than Y
OLLIVIER & CO New Zealand Projections
NZTM Sheets & Names
• Similar concept to NZMG
• Portrait A1 1:50,000 base (prime rectangle)
• Corrected row x col numbering order
• 1:500 - 1:50,000 series
• height/width ratio 3:2
OLLIVIER & CO New Zealand Projections
Using NZ Map Projectionsin ArcGIS
Help! My data doesn’t line up
Adapted by permission from an OZRI 2002 presentation by Rose Read, Brisbane ESRI Australia
OLLIVIER & CO New Zealand Projections
Why not?• Do any layers have unknown coordinate systems?• Are layers from different datums?
• If the datasets differ by thousands or millions of units* Usually the data is in completely different coordinate systems
• All projection parameter files are correct at ArcGIS 9.0* Including North or South Island Yard Grids
•If two datasets are close ( < 200 metres )*Usually different geographic coordinate systems (datums), or
* WGS84 is not recognised as equivalent to NZGD2000, or
*an approximate datum conversion is being used
OLLIVIER & CO New Zealand Projections
Checking for layers with an unknown coordinate system
• A layer does not line up with the other layers
• Open Data Frame Properties
• Click Coordinate System tab...
OLLIVIER & CO New Zealand Projections
Checking the coordinate systems of the layers
• Expand Layers.• Each layer is listed.• Select a layer to see its
coordinate system name.• Road Centrelines projection is
unknown.• Data Frame projection defaults
to first layer added.
OLLIVIER & CO New Zealand Projections
Using ArcMap to identify the unknown coordinate system
• Set the Data Frame to a new coordinate system and see if the known datasets line up with the unknown dataset
If it works…•Remove the layer from ArcMap•Assign the coordinate system of the Data Frame to the layer in ArcCatalog or ArcToolbox•Add the layer back into ArcMap
OLLIVIER & CO New Zealand Projections
Geodatabase Coordinate Systems
• Properties• Fields tab• Click on ‘Shape’• Spatial Reference is listed
below
OLLIVIER & CO New Zealand Projections
Shapefile Coordinate Systems
• Properties• Fields tab• Click on ‘Shape’• Spatial Reference
is listed below
OLLIVIER & CO New Zealand Projections
Raster Coordinate Systems
• Properties• Spatial Reference tab• Images will be reprojected on
the fly, it may be slower, but it’s a cool feature
• Projection and datum data is kept for file based images in the *.aux file.
• The world file is obsolete.
OLLIVIER & CO New Zealand Projections
Coverage Coordinate Systems
• Properties• Projection tab• Can show spatial reference or
PRJ file format• Update your projection files to
include a DATUM• You cannot define
NZGD2000 as a datum at 9.0 so NTv2 cannot be used to transform back to Geodetic
OLLIVIER & CO New Zealand Projections
Does it work now?NO
• Research for more information:• Did someone else use an approximate conversion?• Data provider or source or age?• Captured at different resolutions?• Similar data types (Map series)?• Common coordinate systems used in the area?• Check the coordinate extents, each NZ series uses a
different range as a hint.
OLLIVIER & CO New Zealand Projections
Removing a coordinate system
• Start ArcCatalog• Browse to the dataset and
open its Properties• Locate the Coordinate System
tab• Select Clear• Click OK
OLLIVIER & CO New Zealand Projections
“Missing spatial reference” message
• When you add data to ArcMap, a warning appears that the data cannot be re-projected.
• Do not ignore this if you want to use a transform or projection
OLLIVIER & CO New Zealand Projections
What is GCS_Assumed_Geographic_1?
• Assigned to shapefiles if the coordinate values are between +/-180 for X and +/-90 for Y
• Allows data to be approximately projected in ArcMap• Has no defined transformations, uses a sphere• Just as bad as no projection in practice• Likely to occur in GPS data from other software
Never leave the datum undefined for lat/long layers
OLLIVIER & CO New Zealand Projections
Warning: “geographic coordinate system differs”…
OLLIVIER & CO New Zealand Projections
Setting a geographic transformation (to change the Datum)
• Open Data Frame Properties
• Select Coordinate System tab
• Click Transformations
OLLIVIER & CO New Zealand Projections
Setting a transformation, continued
• Convert from:– Lists the geographic
coordinate systems of the layers in the data frame
• Into:– Data Frame’s geographic
coordinate system
• Using:– Predefined transformations
OLLIVIER & CO New Zealand Projections
Name LINZ parameters
New_Zealand_1949_To_NZGD_2000_1_3PAR 54.4 -20.1 183.1
New_Zealand_1949_To_NZGD_2000_2_7PAR -59.47 -5.04 187.44 -0.47 0.10 -1.024 -4.5993
New_Zealand_1949_To_NZGD_2000_3_NTv2 20km Distortion Grid
Type Accuracy
3 parameter 5 metres
7 parameter 4 metres
Distortion Grid 0.2 metres
If your data is at a resolution of less than the accuracy of the transform, then its nearly OK
ArcGIS Datum Transformation Parameter Files
OLLIVIER & CO New Zealand Projections
7 Parameter coverage conversion example
/* nzmg_nztm7.prj/* APPROXIMATE NZMG to NZTM (~4 metre error)input projection NEWZEALAND_GRIDunits metersdatum USER_DEFINED 59.47 -5.04 187.44 -0.47 0.10 -1.024 -4.5993parametersoutputprojection TRANSVERSEunits metersdatum WGS84 SEVENparameters0.9996173 00 0000 00 00160000010000000end
ArcInfo Projection Files
OLLIVIER & CO New Zealand Projections
Datum Adjustment outside ArcGIS
• Equivalent NTv2 adjustment can be done using Arc: ADJUST with a cover of links created from the distortion grid. User group download script.
• ArcView 3.2 has an extension datum.avx for 3 parameter conversions. Warning: it uses a table of out-of-date 7 parameter figures, but only uses a 3 parameter equation, correct them first.
• LINZ have a stand-alone nzgd2000.exe to handle single values and bulk text files.
OLLIVIER & CO New Zealand Projections
Rearrange the parameter lists
• Re-arrange the coordinate folders to make them easier to find, move all the others to an Other.
• Different parameter conventions between ArcInfo & ArcGIS, especially conversion units for yards
OLLIVIER & CO New Zealand Projections
ArcIMS
Can set a datum manually in AXL
See Australian Spatial newsletter
OLLIVIER & CO New Zealand Projections
Reprojecting Images- a key barrier to change
• All new images will default to NZTM (photo control)• Converting old images is more difficult than vector, as
last year’s paper by Dave Horry• Tiled images have to be merged with adjacent sheets• Resampling degrades colours and resolution• No quality assurance on the results• Just reproject the world file??
OLLIVIER & CO New Zealand Projections
Discussion
FAQ at www.ollivier.co.nz/projection