There are a number of
great online GIS ( Geographic Information System ) solutions
out there, ranging from Mappoint to Mapquest to some cool
PHP projects. However, it is still nice to be able to be able
plot you own data and put some basic mapping utilities on
your site with a minimum of fuss and bother. Over the next
couple of articles I will hopefully be able to give you a
few pointers on how to create you own basic GIS system for
free. I do not claim to be a GIS expert, and there may be
better ways to do things, but hopefully these articles will
give you a start in the right direction.
In this article we will start with the absolute
basics - plotting a single location on a pre drawn map of
the world. Later articles will go into drawing vector maps,
drawing roads and rivers and labeling objects all using the
principles in this article.
For this article you will need:
PHP with GD installed
A pair of scissors
An adult to help you
Finding your place in this world
To plot an object on any map, be it a house
or a herd of caramelized Wombats, the object needs to be geocoded
- that is have a map coordinate allocated to it. For all examples
we used a standard longitude (long) and latitude (lat) coordinate.
There a numerous other coordinate systems out there - some
a lot more accurate, but this is the most common, the easiest
to plot and suits our needs. The downside of directly plotting
long/lat coordinates is that you will get distortion as you
near the poles. However, for our uses this doesn't pose a
The two coordinates of a longitude and latitude
refer to the angle in degrees from the equatorial plane of
the earth, both up and down. Longitude lines extend from pole
to pole giving us an "X" coordinate, Latitude lines
give us the "Y" coordinate. These can either be
written as a decimal value ( ie 23.323232 ) or as degrees,
minutes and seconds ( D'M'S ). For storing our coordinates
we used the decimal version as it's a lot easier and saves
a lot of processing time. There are a number of sites with
( degrees, minutes and seconds ) to the decimal format. I
happen to use a Sharp EL-546 scientific calculator which has
the function built in.
There are a number of ways to finding the
long/lat of a point in the world, however a quick and easy
way is to used Microsofts online GIS service Mappoint ( www.mappoint.com
). Find your location using Mappoint search utilities and
make sure it is in the center of the map. Right mouse click
on the map, and you will find long/lat coordinates embedded
in the URL of the map image. It is also worth looking at www.geogratis.com,
plus a number of government operated environmental sites will
have map data which you can download for free. Those of you
who have Mapinfo will also find very useful geocoded data
on the sample discs which come with it.
The point we are plotting for this article
is my 64 bedroom mansion, located in the bustling urban metropolis
which is Prince Edward Island, Canada. Using Mappoint I have
discovered my location is:
We now need a base map to plot our point
on. In future articles we will generate this ourselves, however
to start we will use a simple pre drawn JPG file. The base
map we are using is called earth_310.jpg, and is and view
of the earth in what is called a Cylindrical projection.
The original of this image can be found in
various forms all over the web in various sizes and scales.
A cylindrical projection is the simplest projection to plot
long/lat coordinates onto, again with a minimum of conversion
needed reducing processor overhead. For our purposes you can
simply right click the image and "Save picture as ..."
to your hard drive.
The base map is scaled to 310x155 for ease of use, but you
can rescale this map to any size.
We are now ready to generate the code to plot our point.
Plots and schemes
The basic steps for generating the map are:
Load the background map.
Convert and scale the long/lat coordinates to screen coordinates.
Plot the point.
Return the finished map as an image.
To convert the long/lat to screen coordinate
we have a created a function called getlocationcoords.
This takes the longitude and latitude coordinates plus the
size of the base map and return the screen coordinates in
an associative array.
are calculated from the size of the background image. In future
projects these variables are used scale the map and set the
function getlocationcoords($lat, $lon, $width, $height)
$x = (($lon + 180) * ($width / 360));
$y = ((($lat * -1) + 90) * ($height / 180));
Once the coordinates have been converted
it's as simple as drawing a rectangle on the base map using
the returned array to mark our location.
So, the code to create the map:
<?php // These are the coordinates
the location we wish to plot.
// These are being passed in the URL, but we will set them
to a default if nothing is passed.
if(empty($long))$long = -63.10774861954596;
if(empty($lat)) $lat = 46.2899306519141;
// First we load the background/base map. We assume it's located
in same dir as the script.
// This can be any format but we are using JPG in this example
// We will also allocate the color for the marker
$im = imagecreatefromjpeg("earth_310.jpg");
$red = imagecolorallocate ($im, 255,0,0);
// Next need to find the base image size.
// We need these variables to be able scale the long/lat coordinates.
$scale_x = imagesx($im);
$scale_y = imagesy($im);
// Now we convert the long/lat coordinates into screen coordinates
$pt = getlocationcoords($lat, $long, $scale_x, $scale_y);
// Now mark the point on the map using a red 4 pixel rectangle
// Return the map image. We are using a PNG format as it gives
better final image quality than a JPG
header ("Content-type: image/png");
To load the map call the PHP code from an
IMG tag, passing the longitude and latitude in the long and
You could easily modify this code to read
long/lat coordinates from a database or a delimited text file
and plot multiple points.
This is a very basic script. However the basic principle
will allow you to draw very complicated maps.
The important part is the getlocationcoords
function - once you have geocoded your data and have the
routines to plot the points on the screen, the sky's the