G.Projector — Global Map Projector

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 be able to open TIFF images. Furthermore, you can open ground overlay images identified within many 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 window that shows only the lon-lat grid and continental outlines. You can later import a map into the projection you are working with.

But if you apply an bitmapped input map image at the start, 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 asking you to enter the lines of longitudes and latitudes that define the edges of the image.

If, however, you select a KML or KMZ file, G.Projector will parse the information within that 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 understand rotated ground overlay images.

The G.Projector distribution includes a folder called "sample_maps" that holds four sample images: two global maps of Earth and one of Mars, plus a KMZ file that 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 you use larger input images — maps 5000×2500 pixels or larger — from other sources, please be aware that G.Projector may encounter memory problems if you have more than one map window open at a time. The program may hang or report a "heap space" error if the source image is in excess of 10,000×5000 pixels.

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 that lists the over 100 projections currently available. Next are two text fields for entering the longitude and latitude at which the map should be centered. However, many of the projections in G.Projector cannot be centered off the equator, and in those cases the latitude field will be hidden. A central latitude is always required .

Several of the projections also have an extra parameter or two. When one of these projections is chosen, additional text input fields, checkboxes, and/or pop-up menus will be appended to the line of controls. For example, the Equirectangular projection has a parameter called "Standard Parallel" (the latitude at which east-west distances are correctly scaled); a text field for entering that value will appear when that projection is chosen.

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 and/or political boundaries. 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.) Two 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, CNOB or SHP overlay files located in this folder will be automatically added to the menu or overlay choices when G.Projector is started. SO if you have overlay files of your own that you would like to use, just place them in this folder.

If the overlay you have chosen is a shapefile (it has a .SHP extension), then an extra overlay option for "point symbol" is enabled. However, this option is only useful if the particular shapefile is made up of point or multipoint record types.

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 input map image than your initial choice. To do so, select the "Import Map Image..." item in the File menu.

You can also use CNO, CNOB and SHP overlay files located elsewhere on your hard drive than in the standard overlays folder. Select "Import 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 box to be displayed that shows the longitude and latitude of the point 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. Azimuthal projections are re-centered on the click point, while non-azimuthal projections are instead centered on the longtitude of the click point. Interrupted pseudocylindric maps cannot be re-centered at all.


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

Robert Schmunk
NASA Goddard Institute for Space Studies
2880 Broadway
New York, NY 10025 USA

(Updated Oct. 28, 2014)

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