mapdraw

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mapdraw

• ats315

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Todays goal

• Drawing a map in the graphics window!

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Previous Skills

• Basic graphic commands (opening windows, drawing
  lines)
• Reading files
• The while statement
• functions

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The Map Files

• state.ats315.dat
• nice state outlines
• usa_cnty.ats315.dat
• nice county outlines
• rd_int.ats315.dat
• U.S. Interstate Highways
• rivermap.ats315.dat
• major U.S. rivers

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Where are the map files?

• /usr/local/wxp/etc/

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How the map files work

• They are collections of line segments.
• lat1 lon1 lat2 lon2
• You dont know in advance how many line segments
  there will be.

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Reading the map file
while (!feof(fmap)) fscanf (fmap, f f f
f,lat1, lon1, lat2, lon2) / Now do stuff
with this information /
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Reading the map file
while (!feof(fmap)) fscanf (fmap, f f f
f,lat1, lon1, lat2, lon2) / Now do stuff
with this information /

• What to do with these lats and lons is the tricky
  part!

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Why this is tricky

• The map file is full of latitudes and longitudes
  of the line segments that make up the map.
• Your graphics window is NOT expressed in terms of
  latitude and longituderather it is a unit
  square!
• Therefore, you need to transform (lat,lon) into
  (x,y).

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Transforming coordinates

• Is best done in a function.
• Can be done in two functions, but Id prefer to
  see you do it with just one.

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Transformations

• Latitude corresponds to the y direction.
• Longitude corresponds to the x direction.

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Transformations

• Lets focus on LATITUDE first.

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Transformations

• You need some kind of function that converts the
  latitude to a number between 0 and 1.

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Transformations

• More to the point, you need this function to
  convert JUST THE PART OF THE MAP YOU WANT to a
  number between 0 and 1.

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Transformations

• For example, here clearly some latitudes are
  outside the (0,1) range for y, and so they arent
  plotted.

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Transformations

• You could write a function called
  transformlatitude(lat).

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Transformations

• Your northernmost latitude becomes 1.
• YOU get to pick your northernmost latitude!

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Transformations

• Your southernmost latitude becomes 0.
• YOU get to pick your southernmost latitude!

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Transformations

• Could look something like this

float transformlatitude(float lat) float
northernmost, southernmost. y northernmost
40. southernmost 30. y (lat
southernmost) / (northernmost
southernmost) return y
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Transformations

• Interpolates your lat between 0 and 1 if it is
  between northernmost and southernmost

float transformlatitude(float lat) float
northernmost, southernmost. y northernmost
40. southernmost 30. y (lat
southernmost) / (northernmost
southernmost) return y
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Transformations

• Then your map reading program could call
  transformlatitude to get a value on the unit
  square

while (!feof(fmap)) fscanf (fmap, f f f
f,lat1, lon1, lat2, lon2) y1
transformlatitude (lat1) y2 transformlatitude
(lat2) / Now do this with the longitudes as
well! / / Now do stuff with this information
/
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Transformations

• A similar function could be written for
  longitudes then

while (!feof(fmap)) fscanf (fmap, f f f
f,lat1, lon1, lat2, lon2) y1
transformlatitude (lat1) y2 transformlatitude
(lat2) x1 transformlongitude (lon1) x2
transformlongitude (lon2) / Now do stuff with
this information /
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Transformations

• Then decide what you are going to do with this
  new (x1,y1) and (x2,y2)!

while (!feof(fmap)) fscanf (fmap, f f f
f,lat1, lon1, lat2, lon2) y1
transformlatitude (lat1) y2 transformlatitude
(lat2) x1 transformlongitude (lon1) x2
transformlongitude (lon2) / Now do stuff with
this information /
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Ideally

• you really should use just ONE function to
  transform the latitudes and longitudes into x and
  y
• transform(lat,lon,x,y)
• But, to do this, youll need to think VERY
  carefully about which parameters are passed by
  VALUE and which are passed by ADDRESS!

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Which one is it?

• transform(lat,lon,x,y)
• transform(lat,lon,x,y)
• transform(lat,lon,x,y)
• transform(lat,lon,x,y)
• Or even something else????

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Your Assignment

• There is no specific assignment about this
  lecture.
• You DO have to draw a mapwith clippingfor
  Friday.
• Therefore, start working on this NOW!