                      MAP DEMO

   This demonstration is a tool for experimenting      
   with common map projections and great circle paths.
   All of the map projections are drawn by the MAP_SET
   IDL User's Library Procedure. The forward and
   inverse map transformations are built into IDL.
   The projections are described in more detail below.

   You can draw the great circle connecting two 
   selected points or cities, showing both the route 
   and distance.

   A small data base of approximately 50 cities is 
   included. We apologize in advance if your favorite
   city is not included. Inverse map transformations 
   are demonstrated by moving the mouse on the map,
   displaying the latitude and longitude of the 
   selected point. The center of projection may be 
   moved by dragging your mouse from one point on the 
   map to another.


   MENU OPTIONS
   ------------

   File Menu:
      Select "Quit" to exit the Map Demo and return 
      to the IDL Demo main screen.
   
   Edit Menu:
      Select the "Reset" button to set the center 
      latitude, longitude, and rotation to zero.
      The selected projection is then redrawn.

   View Menu:

       Continents
       This menu allows the display of continental
       outlines, filled continents, or continental
       elevations. Elevations are displayed by warping
       a digital elevation grid over the current map. 
       o Select "None" to disable the display of
         continental outlines, filled continents or
         continental elevation data.
       o Select "Outlines" to draw continental outlines.
       o Select "Fill" to display the continents as 
         filled polygons.
       o Select "Low Res Elevations" to set the resolution
         to a two-degree square elevation grid.
       o Select "Medium Res" to set the resolution to a 
         one-degree square elevation grid. 
       o Select "High Res" to use a more accurate, but 
         slower interpolation method.

       Interpolation
       IDL provides two algorithms for interpolating 
       sampled data to maps: MAP_IMAGE, an image space
       algorithm and MAP_PATCH, an object space 
       algorithm. 
       o Image space algorithms perform an inverse 
         map transformation to obtain the latitude and
         longitude coordinates of each screen point. 
       o Object space algorithms operate in the 
         opposite direction, interpolating the mesh 
         described by the data points onto the screen,
         and then filling the resulting polygons.

       Rivers
       Draw rivers on the map.

       Boundaries
       Draw country boundaries on the map. State 
       boundaries for the United States are also
       drawn.

       Cities
       Draw selected cities on the map.

       Isotropy
       Non-scale maps may be displayed isotropically, 
       with an equal scale in the X and Y directions,
       or with the map scaled to fit the window in
       both the X and Y directions.


   Cities Menu:

       Mark All
       Displays the cities on the current map.

       Find
       Displays the City menu.  Selecting a city from 
       the list displays its location on the map and 
       displays its latitude and longitude at the 
       bottom right of the demo window.


   Great Circles Menu:

       Connect Two Points
       Draw the great circle connecting two points 
       by clicking on the "Connect two points" button,
       and then clicking on the two points. You can
       also connect cities by selecting "Find" from 
       the Cities menu after clicking "Connect two
       points". The distance between the cities along 
       the great circle is also shown.

       Draw
       Draw the great circle along the prime meridian
       or draw the last drawn meridian in a different
       color. 

   About Menu:
       Select "About Maps" for information about the 
       Map Demo.



   FEATURES
   --------

   <<PROJECTION>> list
   Click on the name of the desired projection.  
   Projections supplied by IDL are described in more 
   detail after the FEATURES section.

   <<CENTER LONGITUDE>> slider
   Vary the longitude of the center of the projection.
   Positive longitudes are east of the prime meridian;
   negative longitudes are west of the prime meridian.

   <<CENTER LATITUDE>> slider
   Vary the latitude of the center of the projection.

   <<ROTATION>> slider
   Set the rotation of the earth with respect to 
   the vertical polar axis.

   <<Scale>> field
   If the scale is set to zero, the map is sized to 
   fit the drawing window.  Otherwise, the map is 
   drawn with the designated true scale about the 
   center.  Usable scales range from about 1 million 
   to one (15 miles per inch, or 10Km/Cm), to 300 
   million to one (4700 inches/mile, or 3000 Km/Cm).

   You can also rotate the globe interactively. 
   Position the cursor on the image and, while holding
   down the left mouse button, move the cursor to 
   rotate the globe. Release the mouse button when you
   are satisfied with the position of the globe. 




*********   PROJECTIONS    **********


   AZIMUTHAL PROJECTIONS
   ---------------------
   With azimuthal projections, the UV plane is tangent
   to the globe.  The point of tangency is projected 
   onto the center of the plane and its latitude and 
   longitude are P0lat and P0lon respectively.  Rot 
   is the angle between North and the V-axis.

   Important characteristics of azimuthal maps include
   the fact that directions or azimuths are correct 
   from the center of the projection to any other 
   point and great circles through the center are 
   projected to straight lines on the plane.

   The IDL mapping package includes the following 
   azimuthal projections: Stereographic, Orthographic,
   Gnomonic, Lambert's Azimuthal Equal Area
   and the Azimuthal Equidistant projection.


   STEREOGRAPHIC
   -------------
   The stereographic projection is an azimuthal, 
   true perspective projection with the globe being 
   projected onto the UV plane from the point P on 
   the globe diametrically opposite to the point of 
   UV tangency. The whole globe except P is mapped 
   onto the UV plane. There is, of course, great 
   distortion for regions close to P, since P maps
   to infinity.

   The stereographic projection is commonly used for 
   polar projections (set Center latitude to + or - 90
   degrees). All great or small circles are shown as 
   circular arcs or straight lines.


   ORTHOGRAPHIC
   ------------
   The orthographic projection is an azimuthal 
   perspective projection with point of perspective at
   infinity.  As such, it maps one hemisphere of the 
   globe into the UV plane. Distortions are greatest
   along the rim of the hemisphere where distances and
   land masses are compressed.

   The primary usage is for pictorial views of the 
   Earth, resembling those seen from space.
   All great circles are shown as elliptical arcs or 
   straight lines.


   LAMBERT CONIC
   -------------
   The conic projection in this mapping package is 
   Lambert's conformal conic with two standard 
   parallels. It is constructed by projecting the 
   globe onto a cone passing through two parallels.
   There is additional scaling to achieve 
   conformality. The pole under the cone's apex is
   transformed to a point and the other pole is mapped
   to infinity. The scale is correct along the two 
   standard parallels. Parallels are projected onto 
   circles and meridians onto equally spaced straight
   lines.
 
   For this projection only, the Center Latitude 
   Slider controls the latitude of one standard 
   parallel, and the Center Longitude Slider controls 
   the latitude of the other standard parallel.

   The primary usage is for large-scale mapping of 
   areas of largely east-west extent. 

   LAMBERT'S AZIMUTHAL
   -------------------
   Lambert's cylindrical equal area projection adjusts
   projected distances in order to preserve area. 
   Hence, it is not a true perspective projection.

   Like the stereographic projection, it maps to 
   infinity the point P diametrically opposite the 
   point of tangency. Note also that to preserve area,
   distances between points must become more 
   contracted as the points become closer to P. 
   Lambert's equal area projection has less overall 
   scale variation than the other azimuthal 
   projections in this package.

   Recommended for equal-area mapping of regions near 
   the Equator.

   GNOMIC
   --------
   The Gnomic (or Gnomonic) projection is the 
   perspective, azimuthal projection with point of 
   perspective at the center of the globe. Hence, with 
   the gnomonic projection, the interior of a 
   hemispherical region of the globe is projected to 
   the UV plane with the rim of the hemisphere going 
   to infinity. Except at the center, there is great 
   distortion of shape, area and scale.

   All great circles are shown as straight lines. Used
   by navigators and aviators for determining courses.
   There is too much distortion for many uses.


   AZIMUTHAL EQUIDISTANT
   ---------------------
   The azimuthal equidistant projection is also not a 
   true perspective projection, because it preserves 
   correctly the distances between the tangent point 
   and all other points on the globe. The point P 
   opposite the tangent point is mapped to a circle 
   on the UV plane and hence the whole globe is mapped
   to the plane. There is, of course, infinite
   distortion close to the outer rim of the map, which
   is the circular image of P.

   The polar aspect is used for polar regions. The 
   oblique aspect is used for world maps, centered on 
   important cities.


   SATELLITE
   ---------
   The satellite projection requires your input; the 
   Satellite Projection Parameters dialog appears when 
   you select the Satellite projection. 
   o  The Altitude ranges from 100 km to 15000 km above 
      the Earth. Use it to zoom in on specific areas of 
      the globe.
   o  The Alpha (up) angle refers to the angle of the 
      perspective plane with respect to the globe.
      which the globe is drawn.
   o  The Beta (rotation) angle defines the angle 
      through which to rotate the polar axis, which is 
      vertical by default with Beta set to 0.


   CYLINDRICAL
   -----------
   The cylindrical equidistant projection is one of the 
   simplest projections to construct. If EQ is the 
   equator, this projection simply lays out horizontal 
   and vertical distances on the cylinder to coincide
   numerically with their measurements in latitudes and 
   longitudes on the sphere. Hence, the equidistant 
   cylindrical projection maps the entire globe to a 
   rectangular region bounded by

   -180 <= u <= 180, and -90 <= v <= 90.

   If EQ is the equator, meridians and parallels will be
   equally spaced parallel lines.


   MERCATOR
   --------
   Mercator's projection is partially developed by 
   projecting the globe onto the cylinder from the 
   center of the globe. This is a partial explanation 
   of the projection because vertical distances are 
   subjected to additional transformations to achieve 
   conformality -- that is, local preservation of shapes.
   To properly use the projection, the user should be 
   aware that the two points on the globe 90 degrees 
   from EQ (e.g., the North and South poles in the case
   that EQ is the equator) are mapped to infinite 
   distances.

   MOLLWEIDE
   ---------
   With the Mollweide projection, the central meridian 
   is a straight line, the meridians 90 degrees from the
   central meridian are circular arcs and all other 
   meridians are elliptical arcs.  The Mollweide 
   projection maps the entire globe onto an ellipse in 
   the UV plane. The circular arcs encompass a 
   hemisphere and the rest of the globe is contained in 
   the lines on either side.


   SINUSOIDAL
   ----------
   With the sinusoidal projection, the central meridian
   is a straight line and all other meridians are 
   equally spaced sinusoidal curves. The scaling is true
   along the central meridian as well as along all
   parallels.

   For this projection only, the Center Latitude and 
   Rotation Sliders have no effect.


   AITOFF
   ------

   The Aitoff projection modifies the equatorial aspect 
   of one hemisphere of the azimuthal equidistant 
   projection, described above. Lines parallel to the 
   equator are stretched horizontally and meridian 
   values are doubled, thereby displaying the world as 
   an ellipse with axes in a 2:1 ratio. Both the equator
   and the central meridian are represented at true 
   scale; however, distances measured between the point 
   of tangency and any other point on the map are no 
   longer true to scale.


   HAMMER-AITOFF
   -------------

   The Hammer-Aitoff projection is derived from the 
   equatorial aspect of Lambert's equal area projection,
   limited to a hemisphere (in the same way Aitoff's 
   projection is derived from the equatorial aspect of 
   the azimuthal equidistant projection). The 
   hemisphere is represented inside an ellipse with the 
   rest of the world in the lunes of the ellipse.

   Because the Hammer-Aitoff projection produces an 
   equal area map of the entire globe, it is useful 
   for visual representations of geographically related 
   statistical data and distributions. Astronomers use 
   this projection to show the entire celestial sphere 
   on one map in a way that accurately depicts the 
   relative distribution of the stars in different 
   regions of the sky.


   Alber's Equal Area Conic
   ------------------------

   The Albers Equal-Area Conic is like most other 
   conics in that meridians are equally spaced radii, 
   parallels are concentric arcs of circles and scale is
   constant along any parallel. To maintain equal area, 
   the scale factor along meridians is the reciprocal of
   the scale factor along parallels, with the scale 
   along the parallels between the two standard 
   parallels set too small, and the scale beyond the 
   standard parallels set too large. Standard parallels 
   are correct in scale along the parallel, as well as 
   in every direction. 

   The Albers projection is particularly useful for 
   predominantly east-west regions. Any keywords for 
   the Lambert conformal conic also apply to the Albers 
   conic projection.


   Transverse Mercator
   -------------------

   The Transverse Mercator (also called the UTM, and 
   Gauss-Krueger in Europe) projection rotates the 
   equator of the Mercator projection 90 degrees so that
   it follows a specified central meridian. In other 
   words, the Transverse Mercator involves projecting 
   the Earth onto a cylinder which is always in contact 
   with a meridian instead of with the Equator.

   The central meridian intersects two meridians and the
   Equator at right angles; these four lines are 
   straight. All other meridians and parallels are 
   complex curves which are concave toward the central 
   meridian. Shape is true only within small areas and 
   the areas increase in size as they move away from the
   central meridian. Most other IDL projections are 
   scaled in the range of +/- 1 to +/- 2 Pi; the UV 
   plane of the Transverse Mercator projection is scaled
   in meters. The conformal nature of this projection 
   and its use of the meridian makes it useful for 
   north-south regions.


   Miller Cylindrical
   ------------------

   The Miller projection is a simple mathematical 
   modification of the Mercator projection, 
   incorporating some aspects of cylindrical 
   projections. It is not equal-area, conformal or 
   equidistant along the meridians. Meridians are 
   equidistant from each other, but latitude parallels 
   are spaced farther apart as they move away from the 
   Equator, thereby keeping shape and area distortion to
   a minimum. The meridians and parallels intersect each
   other at right angles, with the poles shown as 
   straight lines. The Equator is the only line shown 
   true to scale and free of distortion.
