121 lines
5.5 KiB
Markdown
121 lines
5.5 KiB
Markdown
---
|
|
layout: tutorial_v2
|
|
title: Non-geographical maps
|
|
---
|
|
|
|
<style>
|
|
iframe {
|
|
border: 1px solid #ccc;
|
|
border-radius: 5px;
|
|
}
|
|
</style>
|
|
|
|
## Not of this earth
|
|
|
|
Sometimes, maps do not represent things on the surface of the earth and, as such, do not have a concept of geographical latitude and geographical longitude. Most times this refers to big scanned images, such as game maps.
|
|
|
|
For this tutorial we've picked a starmap from Star Control II, a game that is now available as the [open-source project The Ur-Quan Masters](https://en.wikipedia.org/wiki/Star_Control_II#The_Ur-Quan_Masters). These maps were made with a [tool to read the open-source data files](http://www.highprogrammer.com/alan/games/video/uqm/index.html) of the game, and look like this:
|
|
|
|
<center>
|
|
<img src="uqm_map_400px.png" style="border: 1px solid #ccc; border-radius: 5px" /><br/>
|
|
</center>
|
|
|
|
The game has a built-in square coordinate system, as can be seen in the corners. This will allow us to establish a coordinate system.
|
|
|
|
<center>
|
|
<img src="uqm_map_detail.png" style="border: 1px solid #ccc; border-radius: 5px" /><br/>
|
|
</center>
|
|
|
|
|
|
## CRS.Simple
|
|
|
|
**CRS** stands for [coordinate reference system](https://en.wikipedia.org/wiki/Spatial_reference_system), a term used by geographers to explain what the coordinates mean in a coordinate vector. For example, `[15, 60]` represents a point in the indian ocean if using latitude-longitude on the earth, or the solar system Krueger-Z in our starmap.
|
|
|
|
A Leaflet map has one CRS (and *one* CRS *only*), that can be changed when creating the map. For our game map we'll use `CRS.Simple`, which represents a square grid:
|
|
|
|
var map = L.map('map', {
|
|
crs: L.CRS.Simple
|
|
});
|
|
|
|
Then we can just add a `L.ImageOverlay` with the starmap image and its *approximate* bounds:
|
|
|
|
var bounds = [[0,0], [1000,1000]];
|
|
var image = L.imageOverlay('uqm_map_full.png', bounds).addTo(map);
|
|
|
|
And show the whole map:
|
|
|
|
map.fitBounds(bounds);
|
|
|
|
<iframe src='crs-simple-example1.html' width='616' height='416'></iframe>
|
|
|
|
You can see [this example](crs-simple-example1.html) stand-alone. It doesn't quite work, as we cannot see the whole map after doing a `fitBounds()`.
|
|
|
|
|
|
## Common gotchas in CRS.Simple maps
|
|
|
|
In the default Leaflet CRS, `CRS.Earth`, 360 degrees of longitude are mapped to 256 horizontal pixels (at zoom level 0) and approximately 170 degrees of latitude are mapped to 256 vertical pixels (at zoom level 0).
|
|
|
|
In a `CRS.Simple`, one horizontal map unit is mapped to one horizontal pixel, and *idem* with vertical. This means that the whole map is about 1000x1000 pixels big and won't fit in our HTML container. Luckily, we can set `minZoom` to values lower than zero:
|
|
|
|
var map = L.map('map', {
|
|
crs: L.CRS.Simple,
|
|
minZoom: -5
|
|
});
|
|
|
|
### Pixels vs. map units
|
|
|
|
One common mistake when using `CRS.Simple` is assuming that the map units equal image pixels. In this case, the map covers 1000x1000 units, but the image is 2315x2315 pixels big. Different cases will call for one pixel = one map unit, or 64 pixels = one map unit, or anything. **Think in map units** in a grid, and then add your layers (`L.ImageOverlay`s, `L.Marker`s and so on) accordingly.
|
|
|
|
In fact, the image we're using covers more than 1000 map units - there is a sizable margin. Measuring how many pixels there are between the 0 and 1000 coordinates, and extrapolating, we can have the right coordinate bounds for this image:
|
|
|
|
var bounds = [[-26.5,-25], [1021.5,1023]];
|
|
var image = L.imageOverlay('uqm_map_full.png', bounds).addTo(map);
|
|
|
|
While we're at it, let's add some markers:
|
|
|
|
var sol = L.latLng([ 145, 175.2 ]);
|
|
L.marker(sol).addTo(map);
|
|
map.setView( [70, 120], 1);
|
|
|
|
<iframe src='crs-simple-example2.html' width='616' height='416'></iframe>
|
|
|
|
You can see [this example](crs-simple-example2.html) stand-alone.
|
|
|
|
### This is not the `LatLng` you're looking for
|
|
|
|
You'll notice that Sol is at coordinates `[145,175]` instead of `[175,145]`, and the same happens with the map center. Coordinates in `CRS.Simple` take the form of `[y, x]` instead of `[x, y]`, in the same way Leaflet uses `[lat, lng]` instead of `[lng, lat]`.
|
|
|
|
<small>(In technical terms, Leaflet prefers to use [`[northing, easting]`](https://en.wikipedia.org/wiki/Easting_and_northing) over `[easting, northing]` - the first coordinate in a coordinate pair points "north" and the second points "east")</small>
|
|
|
|
The debate about whether `[lng, lat]` or `[lat, lng]` or `[y, x]` or `[x, y]` [is not new, and there is no clear consensus](http://www.macwright.org/lonlat/). This lack of consensus is why Leaflet has a class named `L.LatLng` instead of the more confusion-prone `L.Coordinate`.
|
|
|
|
If working with `[y, x]` coordinates with something named `L.LatLng` doesn't make much sense to you, you can easily create wrappers for them:
|
|
|
|
var yx = L.latLng;
|
|
|
|
var xy = function(x, y) {
|
|
if (L.Util.isArray(x)) { // When doing xy([x, y]);
|
|
return yx(x[1], x[0]);
|
|
}
|
|
return yx(y, x); // When doing xy(x, y);
|
|
};
|
|
|
|
Now we can add a few stars and even a navigation line with `[x, y]` coordinates:
|
|
|
|
var sol = xy(175.2, 145.0);
|
|
var mizar = xy( 41.6, 130.1);
|
|
var kruegerZ = xy( 13.4, 56.5);
|
|
var deneb = xy(218.7, 8.3);
|
|
|
|
L.marker( sol).addTo(map).bindPopup( 'Sol');
|
|
L.marker( mizar).addTo(map).bindPopup( 'Mizar');
|
|
L.marker(kruegerZ).addTo(map).bindPopup('Krueger-Z');
|
|
L.marker( deneb).addTo(map).bindPopup( 'Deneb');
|
|
|
|
var travel = L.polyline([sol, deneb]).addTo(map);
|
|
|
|
The [resulting map](crs-simple-example3.html) looks pretty much the same, but the code is a bit more readable.
|
|
|
|
<iframe src='crs-simple-example3.html' width='616' height='416'></iframe>
|
|
|