G.Projector 3 — Map Projection Explorer

User's Guide

G.Projector is an interactive tool for exploring map projections. It takes an input map image, typically in Equirectangular form (also called Cylindrical Equidistant), and transforms it to one of over 200 other projections. Continental or political outlines and/or location symbols can also can be drawn overlaying the projected map image.

Input Map Images

When you first launch G.Projector, you are presented with a file dialog asking you to select a source input map. The bitmap image formats that you can use 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 that are identified within many OpenGIS KML and KMZ files, provided that they include Equirectangular maps.

What other constraints are there on your choice?

First, you don't have to immediately select an input map. You may click the Cancel button in the file dialog and just work with a map projection window that shows only a graticule (a longitude-latitude grid) and any selected overlays. At some later time, you could import a map image.

But if you select a bitmap input map image at the start, then the image must be of a map projection that G.Projector is able to read and use for input. After the image file has been selected, a dialog will appear like that in Fig. 1 below, asking you to identify what the input projection is and to specify any parameter values needed to interpret that projection.

Screencap showing the input map options dialog

Figure 1: The input map options dialog, shown for an Equirectangular map projection.

Although G.Projector is capable of creating maps in over 200 projections, it is able to use just a handful of map projections for input. In addition to the standard Equirectangular form, it can also import an Aitoff, Cylindrical Equal-Area, Hammer, Robinson, or Winkel Tripel input map image. But note that implementations of the Robinson vary, so G.Projector may not handle a particular input Robinson map well. Whatever the input image's map projection, it should have no border or extra margins around the edges.

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

The G.Projector distribution includes a folder called sample_maps that contains a few sample input maps — two global map images of Earth and one of Mars — plus a KMZ file that includes a topographic image of just the United States. These are relatively small image files, the largest being 1800×900 pixels and size about 350 kB. The maximum image size that G.Projector will accept for input is calculated based on the available memory on your computer. If you use larger input images — maps 5000×2500 pixels or larger — from other sources, then please be aware that G.Projector may suffer memory problems. Memory constraints could also become an issue if you have more than one map window open at the same time. G.Projector might report a "heap space" error if memory problems occur, or it could simply "hang" (become unresponsive) and require that you force-quit the app.

Map Windows

After the initial open-file and input-options dialogs, G.Projector displays a window showing the map. When G.Projector is launched for the first time, the display is of the Orthographic projection, centered on New York City, as shown in Fig. 2. A controls palette (or floating window) is also visible, where you can change the displayed projection and its parameters. The choice of initial projection and centering, as well as other parameters, can be changed in G.Projector's preferences.

Screencap of sample Orthographic map and the projection controls palette

Figure 2: The initial map display window and the projection controls palette, with the Orthographic projection selected.

The map projection controls includes at top a drop-down menu to choose from the numerous available projections. Below that menu are two text fields for entering the coordinates at which the map is be centered, but one or both of these may be disabled if the projection does not allow for specifying that value. Most of the available projections allow you to specify the central longitude, and many also allow for specifying the latitude. If the projection does not accept a latitude — likely because it is a global projection always centered on the equator — then the latitude field will be disabled.

Several projections may have additional parameters. When one of these projections is chosen, additional text input fields, checkboxes, and/or drop-down menus appear in a third line of the projection tab controls. In Fig. 2, the Orthographic projection controls included a text field for specifying its "edge radius", the angular distance between the map center and the top or bottom edge.

Screencap of sample Mollweide map and the projection controls palette

Figure 3: A map window showing the Mollweide (Oblique) map projection, with rotation and scaling controls in the projection controls palette.

Fig. 3 shows another example of the projection controls in which the "Mollweide (Oblique)" projection has been selected. The palette includes an additional text-field for a parameter called the "third rotation" (the angle at which the central meridian is rotated from the vertical) and a checkbox for applying a specific scaling factor.

In addition to the projection controls, there are several other controls palettes, whose visibility is managed via the menu items in the Window menu.

Screencap of the graticule controls palette

Figure 4: The map graticule (lon-lat grid) controls palette.

Fig. 4 shows the second controls palette, which manages how the graticule — the longitude-latitude grid — is drawn, or if it is to be drawn at all. You may specify the spacing of the grid lines and their style, color, and weight. For cylindrical projections, you may also indicate whether you would like the longitude and latitude values of the lines labeled.

Image showing the map overlays controls tab

Figure 5: The map overlays controls palette.

The third controls palette, shown in Fig. 5, allows you to specify whether and how any line or symbol overlays should should also be drawn overlaying the map. Line overlays will most likely be outlines of continents, rivers, and/or political boundaries. When you first start working with G.Projector, it will offer you seven choices of such outline overlays, including a mix of continental coastlines, national borders, and inland water bodies. These seven choices are "built-in" to the G.Projector application. Other overlays may be imported, as explained below.

If you have opted to use a multipoint shapefile overlay, as shown in Fig. 6, then the overlay controls will re-organize a bit. The control for the stroke pattern is replaced by controls that allow you to indicate how the points are marked on the map. Shapefile polyline and multipoint overlays are not part of the default G.Projector distribution but you can import them.

Screencap of global map using a point shapefile overlay

Figure 6: Global map showing use of a multipoint shapefile overlay, with the overlay controls palette.

The fourth controls palette allows you to specify that night-side shading be applied to the map, and if so, how dark that shading should be. The placement of the shadow is controlled by specifying the subsolar point, i.e., the point directly below the sun. All points more than 90° from that point will be in shadow. Since Earth's obliquity is 23.44°, setting a subsolar latitude greater than 23.44° or less than -23.44° would not be realistic. If you are using an input map that evidences the time of year, then there are further limitations on a realistic setting for the subsolar latitude. Fig. 7 demonstrates how the Orthographic map shown in Fig. 2 might appear with such shading.

Screencap of global map with nightside shading

Figure 7: Global map showing use of nightside shading, and showing the shading controls palette.

Screencap of the layout controls palette

Figure 8: The layout controls palette, with map border and background color controls.

The final map controls palette, shown in Fig. 8, provides controls for setting the background color filling the outside areas of the map, and the color and weight of the map's border.


Mouse Click Options

When viewing a map, there are some special mouse click actions you can use:

An alt-click — i.e., clicking while also holding down the "alt" or "option" key — on the map will cause a box to appear that displays the longitude and latitude of the point where you clicked. The box remains visible until you move the mouse/cursor.

A shift-click on the map will cause most map types to be re-centered. Azimuthal and other oblique projections are re-centered on the click point, while cylindrics, pseudocylindrics and polyconics and some others are re-centered on the longitude of the click point. Some maps, such as interrupted maps, cannot be re-centered at all.

For some, but not all map projections, you can also use mouse clicks to zoom in and out. On macOS, use command-click and alt-command-click — on Windows and Linux, use control-click and alt-control-click. This option is available for the Equirectangular projection and also for most azimuthal and conic projectios.


Importing Overlay Files

G.Projector also allows you to import and use overlay files from other sources by using the "Import Overlay..." command in the File menu. The overlay types recognized are CNO and CNOB outline overlays (as used by the Panoply data visualization app) and also the more common SHP shapefiles. The content of a shapefile varies; it could be a polyline overlay or it could be a multipoint file (the latter type is demonstrated in Fig. 6 above). Note that a shapefile that uses a projected grid definition will probably be rejected with an error message when you try to open it.

Screencap of save-for-future-use dialog that appears when importing an overlay

Figure 9: The "save for future use" dialog that appears when you import a new overlay.

As shown in Fig. 9, when you import an external overlay file, G.Projector will ask if you want to save it for future use so that it is available during future sessions without your having to use the import command again.

Please note that if you import a multipoint shapefile overlay with more than about 100,000 points, G.Projector's performance rendering that overlay may be poor.

Screencap of the overlay library panel in the Preferences window

Figure 10: The overlay library panel in the Preferences window. The "minus sign" delete button becomes enabled when you select an imported overlay saved to the appliction support library.

If you have imported an overlay and indicated that it should be saved for future use, but then later decide that you no longer need it, you can remove it from G.Projector's application support library of saved overlays. Open the Preferences window via the menu command and click on the "Overlay Lib" tab at the top of the window. Select the overlay in the list, as demonstrated in Fig. 10, then click the "minus sign" button just below the left side of the list.


Importing Map Image Files

After a map window has been created, you can replace the input map image with another. To do so, select the "Import Map Image..." item in the File menu.

Unlike overlays, when you import a map image, it is only available for use in the current map window. It cannot be saved to a library for easy future use, nor can it be used in another map in the same session without importing the image again in the second (or later) map.

If you decide to remove the input image being used in a map window but not (yet) replace it with another input image, select the "Remove Source Map Image" item in the Edit menu.


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