G.Projector User's Guide

Input Map Images

When you first start G.Projector, you are presented with a file dialog asking you to select a source input map. The basic image formats you can open include BMP, GIF, JPG, and PNG. On some platforms you may also open TIFF images. Furthermore, you can open ground overlay images identified within OpenGIS KML and KMZ files.

What other constraints are there on your choice?

First, you don't have to select an input map at this time. You could click the cancel button in the file dialog and just work on a map projection which shows only the lon-lat grid and continental outlines. Or you could later import an map into the projection you are working with.

But if you do decide at this time to apply an input map and if you select a bitmap image, then (1) it must be in equirectangular form, and (2) it must have no border or margin. After you select a map image, a dialog will appear requiring you to enter the lines of longitude and latitude that mark the edges of the image.

If, however, you select a KML or KMZ file, G.Projector will parse the information within which identifies the map image to use by looking for the "GroundOverlay" tag. The longitude and latitude bounds of the image are also obtained from this tag. But please note that G.Projector does not yet understand rotated ground overlay images.

The G.Projector distribution includes a folder called "sample_maps" which holds four sample images: two global maps of Earth and one of Mars plus a KMZ file which includes a topographical image of just the United States. These are relatively small files, the largest being 1800×900 pixels and size about 350 kB. If use larger input images — maps 3000×1500 pixels or even larger — from other sources, G.Projector may encounter memory problems if you have more than one map window open at a time.

Map Windows

After that first open-file dialog, G.Projector will display a window showing the map, with a lower panel holding several controls to manipulate the map projection and appearance.

The first line of controls affect the map projection. First is a menu which lists the over 80 projections currently available. Next are two text fields for inputting at which longitude and latitude the map should be centered. However, most of the projections in G.Projector cannot be centered off the equator, so in those cases the latitude field will be hidden. A central latitude is used and the input field will be shown for azimuthal projections such as the Orthographic.

Several of the projections also have an extra parameter or two. When one of those projections is chosen, an additional text input field or a checkbox will be appended to the line of controls. For example, the Equirectangular has a parameter called "Standard Parallel". This is the latitude at which east-west distances are correctly scaled.

The second line of plot controls includes controls for choosing how the lines of the longitude-latitude grid should be drawn, if they are drawn at all.

The third and fourth lines of plot controls allows you to specify what other lines should be drawn overlaying the map, if any. These will most likely be outlines of the continent edges. When you first start working with G.Projector, it will offer you seven choices of Earth continent outlines, the difference being the amount of shoreline detail. (One of these overlays also includes national borders.) One of these choices is built into the G.Projector application, but you will find that the other six correspond to files located in the "overlays" folder in the G.Projector distribution. Any CNO or CNOB overlay files located in this folder will be automatically added to the menu or overlay choices when G.Projector is started.

Also in the controls on the right hand side of the third and fourth lines are controls for setting the background color filling the outside areas of the map, and the color and weight of the map's border.

Importing Image and Overlay Files

After a map window has been created, it is still possible to apply a different imput map image than your initial choice. To do so, select the "Import Map Image..." item in the File menu.

You can also use CNO and CNOB overlay files located elsewhere on your hard drive than in the standard overlays folder. Select "Import Outline Overlay..." from the File menu.

Removing an Imported Image

If you have placed an imported image in your map and decide you want to remove but not to replace it, use the "Remove Import Map Image" item in the Edit menu.

Click Options

When viewing a map, there are two special mouse click options you can use:

An "alt-click", i.e., clicking while also holding down the "alt" or "option" key, on the map projection will cause a little box to be displayed which shows the longitude and latitude of where you clicked. This box remains visible until you click on the map again.

A "shift-click" anywhere on the map will cause the map to be re-centered where you clicked. Note: Only azimuthal projections are truly re-centered at that point. Non-azimuthal projections are instead centered on the longtitude of the click point.

Contact

If you questions about G.Projector which are not answered above, please contact the author:

(Updated Nov. 18, 2009)