PMGlobe, version 3.31 3 Aug 2009
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The Imagery menu

The Imagery menu lets you the choose the images used for daylight and night sides of the globe. These can be the ‘Classic’ PMGlobe land and sea views, built-in full-colour images, or images loaded from files (or any mixture of these).

The menu also lets you control whether sunlighting is calculated and shown, and whether full 3-dimensional (3-D) shading is to be used. You can also change the twilight setting and the background colour.

Daylight image
displays a sub-menu with the following options for controlling the image seen on the daylight side of the globe (or for the whole globe if sunlighting is switched off):

Classic
uses the ‘Classic’ PMGlobe land and sea image (a solid colour for each)

Default (full colour)
uses the built-in full-colour NASA Visible Earth[1]  daylight image

Image from file
uses an image from a selected file for the daylight side of the globe (see the next menu choice for details); this option is only available after a file has been selected

Set daylight image file...
chooses (using the Windows file selection dialog) an image which is then automatically selected for display instead of the Classic or Default images.

The image selected should normally be a cylindrical projection of the Earth, twice as wide as high, with the intersection of the Greenwich Meridian (0° longitude) and the Equator (0° latitude) in the centre. It must have a file extension (type) of .jpg, .gif, or .bmp. For best results it should be at least 900 pixels high.

If any other image or projection is used, it will be projected onto the surface of the globe as though it were a cylindrical projection.

Whenever the image is used to refresh the globe, it will be reloaded if its timestamp has changed since the last use – any update to the image file will therefore, in due course, appear on the globe.

Classic land colour
displays a sub-menu which lets you select the colour of the land shown when using the Classic daylight image

Classic water (lake and sea) colour
displays a sub-menu which lets you select the colour of water features shown when using the Classic daylight image.

Night image
displays a sub-menu with the following options for controlling the image seen on the night side of the globe (when sunlighting is selected):

Classic (starlight)
uses the ‘Classic’ PMGlobe land and sea image to create a ‘starlight’ effect (black sea, faint grey land)

Default (city lights)
uses the built-in NASA Visible Earth night-time image

Image from file
uses an image from a selected file (see below) for the night side of the globe; this option is only available after a file has been selected

Reduced daylight
the currently selected daylight image is used, somewhat darkened, for the night side of the globe

Dark daylight
the currently selected daylight image is used, much darkened, for the night side of the globe

None (pitch black)
no image is shown for the night side of the globe

Set night image file...
chooses (using the Windows file selection dialog) an image which is then automatically selected for night-time display instead of the Classic or Default images. The night-time image is selected and used in the same way as the daylight image from file (see above).

Sunlight
When checked, the globe is displayed as though lit by sunlight at the current time of day, with the daylight image being used for the side of the globe facing the sun and the night-time image for the obverse. The images are mixed in the twilight zone.

This lets you see at a glance which parts of the world are in daylight, and which are in the dark. This lighting is (of course) time dependent, so your PC must have the correct time of day and time zone set in order to show the sunlight correctly (right-click on the clock on the Windows task bar to adjust these, or use the Date and Time option in the Control Panel).

If sunlight is not selected, the daylight image will be used for the whole globe.

When sunlight is being shown, the globe will normally be re-drawn (refreshed) at regular intervals, as selected from the Refresh option in the Options menu. This shows how the sunlight moves across the earth. The section on accuracy gives more details.

3-D
This choice adds shading, to give the globe a ‘three-dimensional’ (3-D) appearance. When using sunlight, the shading used indicates the lighting effect of the sun on the earth (and hence is brightest where the sun is overhead, and somewhat darker near the terminator).

If 3-D is selected without selecting sunlight, artificial lighting from the front and top left of the globe is supplied, as though the globe were being lit by overhead room lighting.

Twilight setting
displays a sub-menu which lets you choose how the twilight band is shown on the globe, when sunlight is in effect.

The sun is not a point source of light, and the atmosphere scatters its light, too. Therefore, sunlight is seen on the dark side of the earth for some time before the sun rises and for some time after it sets. You can select to see all, part, or none of this twilight band on the globe.

The twilight band, when shown, is displayed as a graded mix of the daylight and night-time images. Beyond the selected band, the night-time image is used.

You may choose any one of the twilight settings:

Theoretical sunrise/set (0°)
This setting lights the earth as though by a point source at the centre of the sun; the sunrise/set line is shown as though sunlight ceased when the centre of the sun crossed the horizon, and so no twilight can be seen; exactly half the globe is lit. The line between the dark and light sides of the globe is the terminator.

Ideal sunrise/set (0°50’)
This is a practical and generally accepted definition of sunrise and sunset (fifty minutes of angle after the theoretical sunrise/set) which allows for the diameter of the sun and common atmospheric effects. The line seen on the globe joins the points at which the disc of the sun will have just disappeared or be just about to appear, and encloses a very narrow band of twilight.

Civil twilight (6°)
This setting marks the end of ‘civil twilight’ – a convenient point used for legal purposes (for example, when it is too dark to carry out certain tasks). Please note that PMGlobe’s rendering of this line must not be used for any critical or legal decisions; errors and bugs are always possible, and the definition used by PMGlobe (six degrees after the theoretical sunrise/set) may not match the legal definition used in your country.

Naval twilight (12°)
This setting indicates the points where it is dark for all practical purposes: the centre of the sun is twelve degrees below the horizon.

Astronomical twilight (18°)
At this distance from the terminator no effects from sunlight can be detected, and the centre of the sun is eighteen degrees below the horizon.

Background colour
displays a sub-menu which lets you select the colour of the background used for the globe (the area of the PMGlobe window outside the circle of the globe image).

Footnotes:
[1] The NASA Visible Earth image was compiled from images captured by the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on the Terra satellite. Lights on the night side were imaged from Defense Meteorological Satellite Program spacecraft. For details, see: http://earthobservatory.nasa.gov/Newsroom/BlueMarble/

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Copyright (c) IBM Corporation, 2009. All rights reserved. ©
Author: Mike Cowlishaw, mfc@speleotrove.com