On Colour Ramps and City Dashboards

Here are the colour ramps I am using for numeric measures in the recently launched CityDashboard (which by the way now has a new URL – http://citydashboard.org/):

The colours have been designed to be clearly distinguishable from the white text that is on top of them.

Here is the PHP code that I’m using to choose the appropriate colour for each measure, and which I also used to produce the above ramps – the reverse colour and bad value handling is only implemented where I currently needed, ideally these would be implemented for all the ramps:

$na_rgb = 128;

function getGreyRedHex($val, $min, $max, $reverse=false, $processing=false)
{
	$val_0_255 = getNormalisedVal($val, $min, $max);
	$r = 128 + 0.5*intval($val_0_255);
	$g = 128 - 0.5*intval($val_0_255);
	$b = 128 - 0.5*intval($val_0_255); 
	return getHex($r, $g, $b, $processing);
}

function getGreyBlueHex($val, $min, $max, $reverse=false, $processing=false)
{
	$val_0_255 = getNormalisedVal($val, $min, $max);
	$r = 128 - 0.5*intval($val_0_255);
	$g = 128 - 0.5*intval($val_0_255);
	$b = 128 + 0.5*intval($val_0_255); 
	return getHex($r, $g, $b, $processing);
}

function getColdWarmHex($val, $min, $max, $reverse=false, $processing=false)
{
	$val_0_255 = getNormalisedVal($val, $min, $max);
	$r = intval($val_0_255);
	$g = 255 - 2*abs(127.5 - $r); 
	$b = 255 - $r;	
	if ($reverse)
	{
		$r_temp = $r;
		$r = $b;
		$b = $r_temp;
	}	 
	return getHex(0.8*$r, 0.8*$g, 0.8*$b, $processing);
}

function getGreenYellowRedHex($val, $min, $max, $reverse=false, $processing=false)
{
	global $na_rgb;
	if ($val === "n/a") { return getHex($na_rgb, $na_rgb, $na_rgb, $processing); }
	if ($val === "?") { return getHex($na_rgb, $na_rgb, $na_rgb, $processing); }
	$val_0_255 = getNormalisedVal($val, $min, $max);
	$r = intval($val_0_255);
	$g = 255 - intval($val_0_255);
	if ($g > 128) { $g = 128; }	
	$b = 0; 
	return getHex($r, $g, $b, $processing);
}

function getRedGreyGreenHex($val, $min, $max, $reverse=false, $processing=false)
{
	global $na_rgb;
	if ($val === "n/a") { return getHex($na_rgb, $na_rgb, $na_rgb, $processing); }
	$val_0_255 = getNormalisedVal($val, $min, $max);
	$r = 255 - intval($val_0_255);
	$g = intval($val_0_255);
	if ($g > 128) { $g = 128; } 
	$b = 128 - abs(127.5 - $val_0_255);
	return getHex($r, $g, $b, $processing);
}

function getNormalisedVal($val, $min, $max)
{
	if ($val < $min) { $val = $min; }
	if ($val > $max) { $val = $max;	}
	$range = $max - $min;
	return ($val - $min)*(255/$range); 
}

function getHex($r, $g, $b, $processing)
{
	$hex = str_pad(dechex($r), 2, "0", STR_PAD_LEFT) 
		. str_pad(dechex($g), 2, "0", STR_PAD_LEFT) 
		. str_pad(dechex($b), 2, "0", STR_PAD_LEFT);

	if ($processing) { return "FF" . $hex; }
	else { return "#" . $hex; }
}

I’ll be presenting CityDashboard at the forthcoming Wherecamp EU unconference which is taking place in Amsterdam this weekend.

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2 thoughts on “On Colour Ramps and City Dashboards

  1. Ollie,

    Your blends aren’t very good for the most common forms of colour blindness (check out color oracle, freeware). I’d be looking at blends of blues to yellows which are generally much safer.

    Rich

  2. p.s., I should also say I’ve been using your Boris bikes maps to show students as examples of good map design using open data and I really like city dashboard (as a Londoner myself…)

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