O LLIVIER & C O New Zealand Projections Projections Workshop Basic Projection Steps NZ Transverse Mercator Projections in ArcGIS 8.2 Reprojecting Images

OLLIVIER & CO New Zealand Projections

Projections Workshop

• Basic Projection Steps

• NZ Transverse Mercator

• Projections in ArcGIS 8.2

• 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


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 each series• There is no easy or exact conversion between them• NZMG and 1949 circuits are same datum• NZTM and 2000 circuits are same datum• GPS coordinates are in WGS84 ~ NZGD2000


NZTM - New Sheet Layouts

New Zealand Transverse Mercator

Metric Topographic Sheets

1:500 - 1:50,000 series

Useful for NZTM Aerial photo tiles


NZTM Sheet 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 Projections

or

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?• Have you loaded the correct projection files?

• If two datasets are close ( < 200 metres )* Usually different geographic coordinate systems (datums).

• If the datasets differ by thousands or millions of units* Usually the units differ, or

* The data is in completely different coordinate systems, or

* There are critical bugs in the default NZ projection definitions

• Load in the patches from www.gisuser.co.nz or Corax CD

* Never use North or South Island Yard Grids


Do any layers have an unknown coordinate system?

• When zoomed to full extent, 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 the first one doesn’t work, try some other likely candidates


Does it work? YES

• 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


CAD Data Coordinate Systems

• Properties• Spatial Reference tab


Raster Coordinate Systems

• Properties• Spatial Reference tab• Images will be reprojected on

the fly, it may be slower, but it’s a cool feature


Coverage Coordinate Systems

• Properties• Projection tab• Can show spatial reference or

PRJ file format• Update your projection files to

include a DATUM


Does it work now?NO

• Research for more information:• Metadata?• Data provider or source or age?• 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


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 projected in ArcMap• Has no defined transformations


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

ArcGIS Datum Transformation Parameter Files


7 Parameter 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.

• 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.


Update your coordinate systems

• Download the coordinate definition files to overwrite all the NZ files which have the latitude and longitude origins reversed or get from Corax demo CDROM.

• Delete the North Island and South Island definitions because ArcGIS and ArcInfo projections do not work properly for yards. (Convert them to metres and define some Metric Yard parameter files)

• Re-arrange the coordinate folders to make them easier to find, move all the others to an Other.


New Sheet Layouts

New Zealand Transverse Mercator

Metric Topographic Sheets

1:500 - 1:50,000 series

Aerial photographs

Documents

O LLIVIER & C O New Zealand Projections Projections Workshop Basic Projection Steps NZ Transverse Mercator Projections in ArcGIS 8.2 Reprojecting Images