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Steps Towards a Volcanic Ash Advisory Google Maps Mashup Using Met Office Data

Sigh…;-)

Hi Tony,
In looking for authoritative news on the cloud about to tr-ash my holiday, I found this advisory:

http://www.metoffice.gov.uk/aviation/vaac/vaacuk.html

There’s a string of coordinates which appear to mark the extent of the cloud at “FL200” (20000 feet?). E.g. N6343 W01935 is 63.43N 19.35W.

It looks ripe for your skills, and if you could get a map and a how to …

Don’t you realise it’s a race weekend – and if I tinker on such a weekend, I tinker F1data?!;-)

Okay – so here’s a half hour hack (I timed it)…

The advisory data from the Met Office looks like this:

Rather than mess around with any scraping, I just copies and pasted some of the data into a text editor (I use TextWrangler on a Mac) and ran a few regular expressions over it. Starting at the bottom of the list:

- remove end of line characters and replace them with a space;
– remove the “-” and any whist space around it and replace it with a return (new line) character;
– replace the space with a tab character.

The result of the regular expression processing is a two column tab separated list of co-ordinates.

These can then be copied and pasted into a spreadsheet. For rapid prototyping purposes, I pasted the data into a Google spreadsheet because I know I can get a access to a CSV output from there, and I also know that I can get access to a map widget that will plot markers given lat/long data in that environment.

BIG OOPS… The next step was to decode the position data. How to read a Volcanic Ash Advisory gave a couple of clues, and a quick test suggested the data is direction, degrees, minutes, seconds concatenated. I’m guessing that in the general case the degrees are always two digits, the minutes one or two digits and the seconds zero or two digits but in the data I looked at the length was always 5 characters for latitude (dDDMM), 6 for longitude (dDDMSS), so I just worked with those fixed lengths.

Most of the mapping tools I use require lat/long co-ordinates in a decimal format, so a quick check of Stack Overflow turned up a function to convert from degrees, minutes, seconds to the decimal version (Converting latitude and longitude to decimal values).

UPDATE: I think the data format is actually just a decimal format to 2 dp. So the above and code below is all, in this case, nonsense… but I’m going to leave the content here anyway…

I then used a variant of this code to hack a formula using Google Apps script to run the conversion:

Using the apps script function I had defined as a spreadsheet formula, I could convert the lat/long data in the format provided by the advisory note into the more typical digital representation. Highlighting the converted lat/long data and Inserting a Google Maps Gadget gave me a mapped preview over the data.

Here’s the code (needs generalising bearing in the 1/2 digits for minutes; I had a problem with substring (not sure what – I only ever got one character out) which is why I used substr for the parsing):>

//http://stackoverflow.com/questions/1140189/converting-latitude-and-longitude-to-decimal-values

/***************************************
*** BROKEN CODE - left in for reference purposes only ***

****************************************/
function ConvertDMSToDD_BROKEN_STUPID(degrees, minutes, seconds, direction) {
    var dd = degrees + minutes/60 + seconds/(60*60);

    if (direction == "S" || direction == "W") {
        dd = dd * -1;
    } // Don't do anything for N or E
    return dd;
}

function ParseDMS(input) {
  var parts = [];
  if ((input.substring(0)=="N")||(input.substring(0)=="S")){
    parts[0]=parseInt(input.substr(1,2));
    parts[1]=parseInt(input.substr(3,2));
    parts[2]=0;
    parts[3]=input.substring(0);
  } else {
    parts[0]=parseInt(input.substr(1,2));
    parts[1]=parseInt(input.substring(3));
    parts[2]=parseInt(input.substr(4,2));
    parts[3]=input.substring(0);
  }
  var coord = ConvertDMSToDD(parts[0], parts[1], parts[2], parts[3]);
  return coord;
}

You can see the [UPDATE: corrected] spreadsheet here: Volcanic Ash Advisory mapping demo.

I replaced the lat/long conversion formula with one that:
– adds a decimal place after the first two digits;
– adds a minus sign for W/S.

Here’s the corrected formula:

function ParseDMS(input) {
  var parts = [];
  parts[0]=input.substring(0);
  parts[1]=input.substr(1,2)+'.'+input.substr(3);
  var coord = parseFloat(parts[1]);
  if ((parts[0]=='S')||(parts[0]=='W')) coord=coord*-1;
  return coord;
}

So what would be next?

- tidy up the lat/long conversion code so that it works in general case;
– find a way of pulling data into the spreadsheet live.

It would also be interesting to try to take into account altitude data, and then view the data in 3D in something like Google Earth. A heat map view rather than separate markers might also be fun to do (e.g. using HeatMapAPI).

But for now, back to the weekend…

Written by Tony Hirst

April 17, 2010 at 1:23 pm

3 Responses

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  1. Hi Tony – Here’s another interesting take on the question:

    http://flowingdata.com/2010/04/17/the-ash-cloud-and-airport-shutdowns/

    Best wishes to all of you affected by the ash cloud.
    Gary

    Gary Lewis

    April 18, 2010 at 12:13 pm

  2. [...] parts of the OU…) I captured my hacked attempt at working through that problem in the post Steps Towards a Volcanic Ash Advisory Google Maps Mashup Using Met Office Data, which also includes several bad assumptions I made in the original version of the post, so maybe I [...]

  3. Whilst the volcano in question is only producing steam and not ash at the moment, I have just noticed your discussion about Google Earth and Volcanic ash clouds.

    I put together a quick site that generates Google Earth KML files based on Volcanic Ash Advisories and Met Office Ash Zone Predictions, from a library of tools I have built over the last few years related to processing aviation data.

    If the volcano starts up again you can find it http://www.ashtam.co.uk

    You should be able to feed it the url of a recent Volcanic Ash Advisory and it will plot that for you.

    Gary

    Gary Spender

    May 25, 2010 at 5:24 am


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