How to convert geo coordinates to pixels with fixed ratio?

1
votes

I've found many examples with converting coordinates to pixel for specific picture size. But that is not what I need.

I need to convert geo coordinates (latitude and longitude) to screen pixels with defined ratio pixels/degree without binding to picture size. It needed for drawing programmatically few lines on canvas (10-15 kilometers length of each line and 80x80 kilometers whole map).

How can I do that?

2
javamathgeometrygeo
Multiply degrees by your (pixels/degree) constant to retrieve the pixel location. What else is there to it, especially since everything else concerns the specific sphere-to-plane projection you use, but did not mention? – Marko Topolnik
@MarkoTopolnik, yes, I think about this simple method too. But is this method correct? As you know display is flat and earth is round. – BArtWell
Well, either you accept the approximation, or you can't have a fixed pixels/degree ratio. – Marko Topolnik

2 Answers

1
votes

If you want to maintain correct aspect ratio keep in mind that the length of a degree of longitude depends on the latitude: longitudes get shorter as you approach a pole. If you assume a spherical Earth the length of a longitude is multiplied by the cosine of the latitude, e.g. if a degree is 110km on the equator, on 60°N it's 110*cos(60) = 55km.

The earth is pretty flat over distances of less than 100km. Constant scale factors are OK unless you need really high precision, or have to work with geography of Antarctica.

0
votes

If you care about projection (see comments of question) and you use swing to draw on the screen you can use:

http://docs.geotools.org/stable/userguide/unsupported/swing/jmappane.html

Be warned, it is a very heavy library. It is probably faster to use a web page with leaflet (javascript) to get something geographical on the screen.