Cfrgtkky

I use TikZ to draw arrows and labels on maps, generated from other applications.

A minimal example with a global mercator projection might look like this:

documentclass{standalone}

usepackage{tikz}



begin{document}

    begin{tikzpicture}

        node[anchor=south west,inner sep=0] (image) at (0,0) 

            {includegraphics[width=18cm]{my_mercator_world_map.jpg}};

            begin{scope}[x={(image.south east)},y={(image.north west)}]

                draw [->] (0.5,1) -- (0.5421,0.6353);

                node[draw] at (0.2,0.05) {a}; 

        end{scope}

    end{tikzpicture}   

end{document}

It's easy to normalise map coordinates to TikZ coordinates, but sometimes I have many labels or even TikZ generated polygons. It would be very convenient to give the coordinates as geographical coordinates instead of a number between 0 and 1.

Is there a simple way to normalise TikZ coordinates so that they can be given in e.g. decimal lat-lon if I know the exact extent of the map?

In the case above, I'd like to be able to give coordinates as:

draw [->] (0,90) -- (7.578,12.177);

node[draw] at (-108,-81) {a}; 

I also work with local maps, but we can assume that all have a rectangular grid. Map coordinates to any projection would be fantastic, but I guess that is beyond the scope of TikZ.

EDIT

A partly working answer can be found here: tex.stackexchange.com/a/9562/121799

Based on the linked answer, and using coordinate transformations, you can do this:

documentclass[tikz]{standalone}

usetikzlibrary{fpu, calc}

begin{document}

begin{tikzpicture}

    node[anchor=south west,inner sep=0] (image) at (0,0) {includegraphics[width=0.9textwidth]{example-image}};

    begin{scope}[x={($1/360*(image.south east)$)},

        y={($1/180*(image.north west)$)},

        shift={(180,90)},

        ]

        draw[help lines, red, xstep=45,ystep=30] (-180,-90) grid (180,90);

        foreach x in {-180, -135,...,180} { node [anchor=north] at (x,-90) {x}; }

        foreach y in {-90,-60,...,90} { node [anchor=east] at (-180,y) {y}; }

    end{scope}

end{tikzpicture}

end{document}

For more complex transformation, you can define your own coordinate system; look at the tikz manual, section 13.2.5 "Defining New Coordinate System" (at pag. 137 in my copy) and at this example.

Then you can use it, like for example:

documentclass[tikz]{standalone}

usetikzlibrary{fpu, calc}

newcommand{showpoint}[3][5,-15]{

    path (#2) node[circle, fill=blue, inner sep=1pt]{} coordinate(tmp);

    draw [thick, blue, <-] (tmp) -- ++(#1) 

        node[fill=white,]{#3};

    }

begin{document}

begin{tikzpicture}

    node[anchor=south west,inner sep=0] (image) at (0,0) {includegraphics[width=0.9textwidth]{example-image}};

    begin{scope}[x={($1/360*(image.south east)$)},

        y={($1/180*(image.north west)$)},

        shift={(180,90)},

        ]

        draw[help lines, red, xstep=45,ystep=30] (-180,-90) grid (180,90);

        foreach x in {-180, -135,...,180} { node [anchor=north] at (x,-90) {x}; }

        foreach y in {-90,-60,...,90} { node [anchor=east] at (-180,y) {y}; }

        % use them

            showpoint{0,90}{North Pole}

            showpoint{100,90}{still North Pole}

            showpoint{-100,90}{North Pole, again}

            showpoint{-3,40}{Madrid}

            showpoint{-105,40}{Denver}

            showpoint[5,15]{10,44}{Sarzana}

            %

            showpoint{0,0}{geolocation list bug}

    end{scope}

end{tikzpicture}

end{document}