MATLAB: How getting coordinates of geagraphical locations

bubble plotsMapping Toolbox

Hi,
I have the following struct array.
data=struct('Position',[],'Value',[])
data(1).Position='London';
data(1).Value=20;
data(2).Position='Rome';
data(2).Value=100;
What I need is a bubble plot of values field with a map in the background and the bubble must be located where is the geographical location. I know this function http://www.mathworks.com/matlabcentral/fileexchange/27627-plot-google-map but I don't know how to get the coordinates of the locations. Does anyone have an idea?
Thanks
Cheers
Pietro

Best Answer

Pietro - if you know the country code for the city, then you could try the following function which makes use of the Yahoo GeoPlanet Web Service, GeoPlanet YQL Tables.
 % Uses YQL to query the geo.places database for the latitude and longitude
 % of a city, given the city name and country code.
 function [latDegs, lonDegs] = getLatLonViaYql(city,countryCode)
    % build the YQL query
    yqlQuery = sprintf(['select centroid.latitude,centroid.longitude ' ...
                        'from geo.places '                             ...
                        'where text="%s" and country.code="%s" and '   ...
                        'placeTypeName.code="7"'],                     ...
                        city, countryCode);
    % build the URL
    yqlUrl = ['http://query.yahooapis.com/v1/public/yql?q=' yqlQuery];
    % replace blank spaces with %20
    yqlUrl = strrep(yqlUrl,' ','%20');
    % retrieve the data
    xmlData = urlread(yqlUrl);
    % get the latitude and longitude (here we assume that the latitude is
    % within <latitude>XYZ</latitude> and the longitude within
    % <longitude>UVW</longitude> tags)
    posData = regexp(xmlData,'>[0-9.-]+<','match');
    % if exactly two tags match the above regexp then parse the latitude
    % and longitude data
    if length(posData)==2
        latDegs = str2double(char(posData{1}(2:end-1)));
        lonDegs = str2double(char(posData{2}(2:end-1)));
    else
        latDegs = 0;
        lonDegs = 0;
    end
 end
The latitude and longitude positions can then be obtained as
 [latLondon,lonLondon] = getLatLonViaYql('London','GB')
 latLondon =
                 51.507702
 lonLondon =
                  -0.12797
 [latRome,lonRome] = getLatLonViaYql('Rome','IT')
 latRome =
                 41.898399
 lonRome =
                   12.4956
In the YQL query, we grab the "centroid latitude and longitude of a "bounding box" that surrounds the city. (This box is defined by south-west and north-east positions.) Whether this is accurate enough for your application remains to be seen. But constructing two vectors as
 lat = [latLondon ; latRome];
 lon = [lonLondon ; lonRome];
and executing
 plot(lon,lat,'.r','MarkerSize',20)
 plot_google_map
puts the red markers at the positions of London and Rome on the map.
EDIT
Updated the above code to include an additional where condition/clause to ensure that the record returned corresponds to a "Town". Pietro observed that at least two records were being returned for cities like Oslo, Berlin, and Sofia.
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