O LLIVIER & C O New Zealand Projections Projections Workshop 2004 Basic Projection Steps NZ Transverse Mercator Projections in ArcGIS 9.0 Reprojecting

OLLIVIER & CO New Zealand Projections

Projections Workshop 2004

• Basic Projection Steps

• NZ Transverse Mercator

• Projections in ArcGIS 9.0

• Reprojecting Images

• Discussion


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


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


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….


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


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


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


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


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...


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.


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


Geodatabase Coordinate Systems

• Properties• Fields tab• Click on ‘Shape’• Spatial Reference is listed

below


Shapefile Coordinate Systems

• Properties• Fields tab• Click on ‘Shape’• Spatial Reference

is listed below


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.


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


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.


Removing a coordinate system

• Start ArcCatalog• Browse to the dataset and

open its Properties• Locate the Coordinate System

tab• Select Clear• Click OK


“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


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


Warning: “geographic coordinate system differs”…


Setting a geographic transformation (to change the Datum)

• Open Data Frame Properties

• Select Coordinate System tab

• Click Transformations


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


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


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


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.


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


ArcIMS

Can set a datum manually in AXL

See Australian Spatial newsletter


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??


Discussion

FAQ at www.ollivier.co.nz/projection

Documents

O LLIVIER & C O New Zealand Projections Projections Workshop 2004 Basic Projection Steps NZ Transverse Mercator Projections in ArcGIS 9.0 Reprojecting