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- 1. Geographic Computation in Perl Presented by Ian Kluft Silicon Valley Perl March 26, 2015 Santa Clara, California
- 2. Safety warningSafety warning If you think the world is flat, this presentation will make your head explode.
- 3. We use geographic info all the time ● Maps on desktop/mobile ● Car navigation ● Traffic maps ● Find local services – Restaurants – Shopping – Auto shop ● Nearby social peeps ● Find my car ● Speedometer ● Run/bike/walk map ● Astronomy ● Other web/mobile apps
- 4. Maps go way back ● Paper maps go back to ancient times ● Electronic age: GPS makes mapping more accessible ● Now every smartphone has a GPS receiver ● There's more to GPS & maps than most people realize ● Simplified public views try to hide the technical stuff ● We're going to look at some of those details...
- 5. Coordinate system Latitude and longitude ● Any position on Earth has a coordinate ● 3-dimensional positioning uses 3 numbers – Latitude (north/south) – Longitude (east/west) – Altitude (relative to mean sea level/MSL) ● Latitude & longitude are specified in degrees ● Altitude is specified in distance
- 6. Latitude and longitude What kind of degrees? ● Not from a university ● Not related to temperature ● They are angles! ● More precisely, angles from the center of the Earth ● Latitude = 0-90° up or down ● Longitude = 0-180° either way around
- 7. Earth is not a perfect sphere ● Earth is an ellipsoid: bulges out at equator – Centrifugal force from rotation causes this ● Geoid: mathematical models for Earth ellipsoid – Good models come from satellite measurement ● Coordinates must use the same geographic reference system – Otherwise comparing apples and oranges – WGS84 most widely used coordinate system today ● Sea level and altitude are relative to this model
- 8. Lots of angles in Geospatial Data ● Many computations involve angles – Latitude and longitude are angles ● Manipulations use trigonometric functions ● Trig functions use radians – So numbers in degrees must be converted to and from radians – One circle = 360 degrees = 2 * pi radians – Radians derive 2 * pi from distance around circle relative to radius
- 9. Great Circle Routes ● Great Circle: direct route over Earth's surface – Along a line that would go around the sphere – i.e. from San Jose to London crosses Greenland – Flat-projection maps distort great circle routes to look like curves Map generated by the Great Circle Mapper copyright © Karl L. Swartz. http://www.gcmap.com/
- 10. Great Circle Formulas ● See “Aviation Formulary” site http://williams.best.vwh.net/avform.htm ● Distance between points ● Course between points ● Latitude of point on GC ● Lat/lon given radial & dist ● Intersection of 2 radials ● Max latitude of a GC ● GC crossing a parallel ● Intermediate points on a GC ● Cross track error ● Along track distance ● Point known offset from GC
- 11. Great Circle Formula Example Distance between points ● From scratch: d=acos(sin(lat1)*sin(lat2)+cos(lat1)*cos(lat2)*cos(lon1-lon2)) ● In Perl: use Math::Trig 'great_circle_distance'; $distance = great_circle_distance($theta0, $phi0, $theta1, $phi1, [, $rho]); ● Convert angles to radians, theta = longitude, phi = latitude ● Rho is the radius of the Earth – use 6366.71 km – Don't forget to convert result from km to miles if needed
- 12. Perl Modules ● Math::Trig – trigonometry and great circle functions ● GPS::Babel – convert between GPS file formats ● Geo::Formatter - encode & decode lat/lon strings ● Geo::GNIS - parse USGS Geographic Names data ● Geo::GPX - read & write GPX open GPS data files ● Geo::Horizon - distance to visual horizon ● Geo::IP - estimate lat/lon coordinates for IP address
- 13. Perl Modules (continued) ● Geo::Mercator - convert coordinates to flat-projection map ● Geo::Weather - retrieve weather from Weather Channel ● Ham::Locator – convert between lat/lon & maidenhead coords ● Net::GPSD3 - interface to GPSD server on your laptop ● Geo::Google::StaticMaps – Google Maps API wrapper ● Geo::OSM::Tiles - OpenStreetMap tiles ● And many, many more – go search CPAN!
- 14. Example 1 CanSat Search at Black Rock Desert Location: Black Rock Desert, Nevada Problem: Rocket launched, payload missing ● Soka University (Japan) students needed data ● AeroPac club knew rocket landing coordinates ● Turned to Stratofox Aerospace Tracking Team ● Transmitter batteries died before their contact ● I wrote a Perl script to generate a grid search
- 15. Perl Script to Plot Search Grid ● Search area was 3x3 mile parallelogram ● Each side divided into 10 sections, 1584' long ● 10x10 loop projects each computed waypoint ● Command-line controls output to text or GPX ● “gpsbabel” turns GPX into many formats – Raw Garmin waypoint data for upload to GPS – KML for display on Google Earth
- 16. Projecting Search Grid Waypoints ● Nested loop: i = 0-10 over, j = 0-10 up ● Over = 270° heading, up = 330° heading ● Convert lat/lon from degrees to radians ● Use Great Circle projection formula – Compute intermediate point from i “over” – Compute final point from j “up” ● Convert new lat/lon from radians to degrees ● Code available at svperl.org with slides
- 17. Sample code convert feet to radians ● Convert distance in feet to radians over Earth's surface # conversion: distance in feet to radians sub dist_ft2rad { my $ft = shift; my $km = $ft / 3280.8399; # ft/km return $km/6371.0; # divide by FAI standard Earth radius in km }
- 18. Sample code compute waypoint from course & distance sub gc_waypoint { my $lat1 = shift; # latitude (radians) my $lon1 = shift; # longitude (radians) my $tc = shift; # true course (radians) my $d = shift; # distance (radians) my $lat = asin(sin($lat1)*cos($d)+cos($lat1)*sin($d)*cos($tc)); my $dlon = atan2(sin($tc)*sin($d)*cos($lat1),cos($d)-sin($lat1)*sin($lat)); my $lon=fmod($lon1-$dlon+pi,2*pi) - pi; return ( $lat, $lon ); # lat/lon in radians }
- 19. Example code compute coordinates of search grid point # project a waypoint in the search area # shape of a parallelogram with sides at headings 030 (NNE) and 090 (east) # sides are on heading 1 (030 degrees) and heading 2 (090 degrees) # increments are 0-10 # each increment is 1584 ft so that 10 of them is 3 miles sub project_waypoint { my $h1_inc = shift; my $h2_inc = shift; # compute intermediate point on the first side of parallelogram my ( $lat_r1, $lon_r1 ) = gc_waypoint ( deg2rad( $point_start[0]), deg2rad( $point_start[1]), $h1_heading, $rad_per_increment * $h1_inc );
- 20. Example code (continued) compute coordinates of search grid point # compute final projected waypoint in search area my ( $lat_r2, $lon_r2 ) = gc_waypoint ( $lat_r1, $lon_r1, $h2_heading, $rad_per_increment * $h2_inc ); # convert radians to degrees my $lat = rad2deg( $lat_r2 ); my $lon = rad2deg( $lon_r2 ); return ( $lat, $lon ); }
- 21. Search Area Grid Map Result of the script shown on Google Earth
- 22. Result: Success!!! Expected to be worse than needle in a haystack Payload found 2500' west of rocket landing site
- 23. Example 2 Live APRS Balloon Tracking Map Problem: Display APRS tracking data for a balloon and our team who are tracking it ● APRS = Automatic Packet Reporting System, Ham Radio data protocol for transmitting your position ● The balloon and the tracking team all transmit their positions via APRS every minute or so ● Sites to display APRS data on a map don't do what we want
- 24. APRS.FI posted their Perl code ● Popular APRS mapping site APRS.FI uses Perl ● They posted Ham::APRS::FAP (“fabulous APRS parser”) on CPAN for all Perl developers ● You need Ham Radio license to transmit APRS ● You do not need Ham Radio license to receive APRS data ● Many volunteers forward APRS packets to APRS-IS servers on the Internet ● Everyone can subscribe to data streams there
- 25. Custom APRS Mapping Script ● Command-line parameter sets callsigns to log ● Subscribe to APRS-IS via Ham::APRS::FAP ● For each incoming line: – Parse line w/ Ham::APRS::FAP – Save data in a per-callsign list – Update output file in GeoRSS format ● Output file URL can be used in near real-time as an input to Google Maps ● Unfortunately, Google Maps dropped GeoRSS in 2014 update
- 26. Balloon Chase on Google Maps ● A balloon on Feb 5, 2011 was lost in the ocean ● Teams chased it until Carmel – others watched
- 27. More Geographic Computation ideas ● Can I see the aurora on my next airline flight? – Compute maximum latitude of its great circle ● Web map API “mashups” ● Social networking proximity ● Nearby USGS GNIS landmarks ● Local searches are “Location based services” – The sky is the limit

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