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First, lets start up SFMAP and read in the defaults file:
prompt% sfmap GEMAPAK-SFMAP>restore defaults/sfmap.nts>Now, display the user interface:
GEMPAK-SFMAP>dis AREA Data area GAREA Graphics area SFPARM Surface parameter list skyc;tmpf;wsym;rmsl;ptnd;dwpf;p03i;brbk:.7:2 DATTIM Date/time SFFILE Surface data file COLORS Color list 1;2;3;25;5;6;7;9 MAP Map color/dash/width 3/1/1 LATLON Line color/dash/width/label/inc 0 TITLE Title color/line/title 1/-2 CLEAR Clear screen flag YES PANEL Panel location/color/dash/width 0 DEVICE Graphics device xw PROJ Map projection/angles/margins nps FILTER Filter data factor yes TEXT Text size/font/width/hw flag 1.3/23//HW SKPMIS Skip missing data flag no CLRBAR Color/ornt/anch/x;y/ln;wd/freq GEMPAK-SFMAP>
Okay, of the variables used in SFMAP, only AREA, GAREA, DATTIM, and SFFILE need to be set. The rest are set to default values. We already know about DATTIM, SFFILE and AREA from our work with SFLIST. Of the remaining new variables, lets start with GAREA.
So, what's the
difference between the GAREA and AREA?
ANSWER: By definition, GAREA is the graphics area, meaning the part of the map that will be plotted. AREA is the data area, meaning the area from which to select the data to plot. Less obvious, is why GEMPAK separates the two:
Sometimes you might want separate control over the region of the map that is plotted, and the region that is covered by data.
A practical example of
this is, when you want to plot a map with complete data coverage,
including the edges, you should make AREA bigger than GAREA. This
can be done by adding a minus "-" to AREA or an asterisk "*" to
GAREA:
AREA = CO- or AREA = WV GAREA = CO GAREA = WV*
GAREA can be specified in some of the same ways as AREA:
You cannot use the
@ symbol to specify GAREA.
Why not? Because GAREA defines the boundaries of the graph. Adding an "@" would force an irregular boundary for the graph. Think of the "@" sign as a way to restrict the selection of data.
The rest of these variables are currently set to defaults by the sfmap.nts file. However, it's a good idea to know how they work, so that later you can change them if you need to.
The map projection directly affects the way the map looks on the screen. Think about the way the continental US looks in the Mercator projection versus the Polar Stereographic projection.
The projection can be specified as a simple map projection, or as a full map projection. Without getting into much detail, specifying a map projection in general usually requires a set of angles. Through the PROJ variable and these angles, in combination with the GAREA variable, you have the flexibility to generate a very specific map projection from within GEMPAK. A full map projection in GEMPAK looks like this:
PROJ = STR/90;-100;0
We know from looking at the phelp proj help file
that
GEMPAK-SFMAP>ph proj
...
The following full map projections may also be specified:
MER (CYL) Mercator
MCD (CYL) Modified Cylindrical Equidistant
STR (AZM) Polar Stereographic
...
In addition, three angles MUST be specified in PROJ. The angles
have the following meanings for the different projection classes:
CYL The angles are not used. A default value for angle2,
polon is used.
AZM angle1 -- latitude of the projection's point of tangency.
angle2 -- longitude of the projection's point of tangency.
angle3 -- not used.
...
STR is the Polar Stereographic projection. We see that it
belongs to the (AZM) family. That means it's an azimuthal
projection. For an azimuthal projection, you must specify two
angles. Although the third angle is not used, GEMPAK still expects
to see a 0 in its place.
The first angle is the standard latitude through which the projection plane passes. The second angle is the central longitude. This is the longitude that lies parallel to the grid columns. For a view of North America, STR/90;-100;0 is a good choice.
If this seems a little complicated, you're right, it is. Often
you will find that you don't need to specifiy the full map
projection. GEMPAK comes equiped with a few simple map
projections that will work with standard graphical areas
(mostly over the US). From the phelp proj help file,
they are:
The following simple map projections may be specified:
MER Mercator
NPS North Polar Stereographic
SPS South Polar Stereographic
LCC Northern Hemisphere Lambert Conic Conformal
SCC Southern Hemisphere Lambert Conic Conformal
CED Cylindrical Equidistant
MCD Modified Cylindrical Equidistant
UTM Universal Transverse Mercator
NOR North Orthographic
SOR South Orthographic
To use a simple map projection, simply ask for it:
PROJ = NPS
Here's an example of the NPS simple map projection. The NPS specification is really just an abbreviation for the full map projection STR/90;-90;0. It looks good over the US, but it won't work well for viewing Africa. For that you'll need to use a full map projection. This is true of all simple map projections.
We talked a little about device drivers back in the Introduction to GEMPAK chapter. Since we are going to create a picture, we need to tell GEMPAK how to show us the picture. The default is set to XW (X-Window). This will cause a window to be opened on your screen, and then the graphic will be drawn to it. There are other ways to view your picture, and they each have their own device drivers. We'll discuss them later in the Device Drivers section.
Each time GEMPAK draws a new map or graph to the X-Window, you
have the option of first clearing the screen. When
CLEAR=YES, whatever is currently in the X-Window (if
anything) is wiped away, and the new graphic is drawn on a blank
screen. If CLEAR=NO, then the screen is not
cleared first, and the new graphic plots on top of the existing
graphic. This is how overlays are constructed in GEMPAK. In fact,
the screen will not clear even if you exit the current GEMPAK
program and start another. This is because of the multiple process
design of GEMPAK. For more information on why this works, see the
Introduction to GEMPAK chapter.
We will practice creating overlays later. For now, lets just keep CLEAR=YES.
They are described in their own section: Map Aesthetics
Let's try it... 
Practice
Exercises!!
The parameter's
position in SFPARM corresponds to placement around station (except
for wind barbs which are always centered on the station)...
For Example if:
sfparm= skyc; tmpf; wsym; rmsl
The parameter's position will correspond to this position chart:
18 14 8 16 22
19 2 10 4 23
12 3 1 5 13
20 6 11 7 24
21 15 9 17 25
SKYC goes in position 1, TMPF in position 2, WSYM in position 3, and RMSL in position 4.
Why in that
order?
ANSWER: The numbered positions correspond to the standard ordering of data on a surface chart.
BACK TO THE PRACTICE EXERCISE: In case you're not a met student, a standard surface chart is arranged more or less like so:
The GEMPAK four letter sfparm abbreviations would therefore be:
TMPF RMSL
WSYM SKYC PTND
DWPF & BRBK P03I
So, the SFPARM setting from the default file is already set for
a standard surface chart. All that remains is to set the other
variables. 
You won't find this variable on any user interface, it is a hidden variable.
The default map is a medium resolution, political boundary map of the world. You can choose an alternate map from the available GEMPAK map data base using $MAPFIL. These maps can be especially useful in overlays to add detail to a GEMPAK map, in order to show more political boundaries, add lakes, or even plate tectonics.
The higher the resolution of the map, the longer it takes to draw. For global maps, low resolution is fine. For small scale maps, high resolution can make an impressive graphic.
GEMPAK "remembers" last map file used. The default is HIPOWO.CIA.
Let's try it... Practice Exercises!!
Let's plot a chart over North America, using a coastal boundary map.
$MAPFIL=LOCONH.GSFsfparm=skyc;tmpc;blnk;blnk;blnk;blnk;dwpcor
sfparm=barb;tmpc;;;;;dwpc
PROJ = STR/90;-100;0
AREA=dset GAREA=10;-125;50;-20
hipowo.cia aka: base
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