World Coordinates¶

The points are recorded in screen coordinates (try adding some points on top of a shapefile and zooming around to see this).

Can you make and record the points in world coordinates?

(Hint: getContext() has a worldToPixel method)

Blackboard¶

The blackboard is used to communicate locations between the co-ordinate tool and the MapGraphic. The important part of a blackboard is that it does not matter which component supplied the locations.

Try this out yourself by import points onto the blackboard from a text file.

Locations¶

The worldToPixel method let you work with coordinates in the world; if you change your maps projection they will be wrong.

How to reproduce:

1. You need to complete the World Coordiantes task above before starting this one
2. Zoom into the west coast of Canada
3. Draw several points with your map graphic tool
4. Change the CRS to 3005 which is a projection specific to this part of the world?

Can you figure out a couple ways to fix this?

Remove the Circles¶

The circles are provided by the map graphic to provide visual feedback. We do this in uDig so people can tell that the tool is actually listening to them.

Can you update the Coordinate Tool so that the circle is removed once the co-ordinate is safely stored on the map blackboard?

(Hint: Look up draw commands in the developers guide; or javadocs )

North Arrow¶

A North Arrow would make an excellent Map Graphic how would you make one?

(Hint: you can check your plan for making a north arrow against the one supplied in uDig)

(Hint If you want to use an Image you can ask your Activator to manage the ImageDescriptor for you)

Check what is stored on the blackboard¶

If you are having trouble with the World and Location tasks above take a moment with the debugger to check the following:

• what is being stored on the blackboard by the co-ordinate tool
• what is being taken of the blackboard by the map graphic

Using CoordinateReferenceSystem and MathTransform¶

The trick to working with spatial data is to understand information is recorded (both the values; and what they mean). GeoTools gives us some of the data structures used to track this information:

• CoordianteReferenceSystem: describes how coordinates are measured
• MathTransform: can be used to convert Coordinates between different coordinate reference systems
• DefaultGeographicCRS.WGS84: a coordinate reference system used to quickly work with lat/lon data

It also provides a couple of utility classes to make working with these ideas easier:

• CRS: can help you work with CoordianteReferenceSystems
• JTS: can help you transform

The GeoTools documentation mentioned below has many valuable example including the following one showing how to transform a Geometry between two coordinate reference systems:

import org.geotools.geometry.jts.JTS;
import org.geotools.referencing.CRS;

CoordinateReferenceSystem sourceCRS = CRS.decode("EPSG:4326");
CoordinateReferenceSystem targetCRS = CRS.decode("EPSG:23032");

MathTransform transform = CRS.findMathTransform(sourceCRS, targetCRS);
Geometry targetGeometry = JTS.transform( sourceGeometry, transform);

References:

• http://docs.geotools.org/latest/userguide/library/referencing/crs.html
• http://docs.geotools.org/latest/userguide/library/api/jts.html

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