Custom units of measure

Coordinates have many different types of unit e.g. latitudes are angles, but eastings are lengths.

There are also many different units in use across the world and across history, for instance most current mapping uses the metre for lengths, but older mapping often used feet or yards.

To avoid confusion about what any given number is intended to represent, all units in PHPCoord are typed as value objects.

In addition to the many built-in units, you can also register and make use of custom units. To do this, first create an implementation class for your unit that extends one of the following base classes:

  • PHPCoord\UnitOfMeasure\Angle\Angle

  • PHPCoord\UnitOfMeasure\Length\Length

  • PHPCoord\UnitOfMeasure\Scale\Scale

  • PHPCoord\UnitOfMeasure\Time\Time

Tip

You only have to implement 4 required methods. All UnitOfMeasure require implementations of __construct(), getUnitName() and getValue(). There is then 1 additional method that is specific to each type of unit, e.g. a custom Angle type would be required to implement an asRadians() method. Your IDE will guide you through what is required, although of course you may reference the existing units as a guide.

Then register it, by calling the appropriate registration function:

  • PHPCoord\UnitOfMeasure\Angle\Angle::registerCustomUnit(string $srid, string $name, string $implementingClassFQCN)

  • PHPCoord\UnitOfMeasure\Length\Length::registerCustomUnit(string $srid, string $name, string $implementingClassFQCN)

  • PHPCoord\UnitOfMeasure\Scale\Scale::registerCustomUnit(string $srid, string $name, string $implementingClassFQCN)

  • PHPCoord\UnitOfMeasure\Time\Time::registerCustomUnit(string $srid, string $name, string $implementingClassFQCN)

Once registered, your custom unit can be used exactly like any other built into PHPCoord.

Tip

A SRID (spatial reference identifier), is a just a unique string that can be used to identify the specific unit in question. The PHPCoord built-in units all use an URN for this purpose, but you can use anything you like as long as it is unique. For a unit, this might include reusing the FQCN.

For example, a custom HalfKilometre unit, could be implemented like this:

<?php
namespace YourApp\Geo\Units;

use PHPCoord\UnitOfMeasure\Length\Length;

class HalfKilometre extends Length {

    public const SRID = 'urn:yourcompany:geo:uom:halfkm';

    private float $length;

    public function __construct(float $length)
    {
        $this->length = $length;
    }

    public function asMetres(): Metre
    {
        return new Metre($this->length * 500);
    }

    public function getValue(): float
    {
        return $this->length;
    }

    public function getUnitName(): string
    {
        return 'Half-kilometre';
    }
}

Length::registerCustomUnit(HalfKilometre::SRID, 'Half-kilometre', HalfKilometre::class);

$value = new HalfKilometre(123);
// or
$value = Angle::makeUnit(123, HalfKilometre::SRID);