Unit Measurement for X and Y coordinates using SetXY and Write

[RationalRabbit]

My projects entail creating an HTML form (with PHP) and submitting that form to a tcpdf file using a blank form as background.
Fields are placed into an array, example below:
$Display=array
(
array("field" => $Group, "X" => 176, "Y" => 30),
array("field" => $Name, "X" => 5, "Y" => 37),
array("field" => $SocialSecurity, "X" => 160, "Y" => 37),
etc.
}
Then:
$pdf->AddPage();

$pdf->setSourceFile('templates/PDF_File.pdf');
$tplIdx = $pdf->importPage(1);
$pdf->useTemplate($tplIdx, 0, 0, 217);

for($i = 0; $i < sizeof($Display2); ++$i)
{
$Xpos = $Display2[$i]["X"];
$Ypos = $Display2[$i]["Y"];

  $pdf->SetXY($Xpos, $Ypos);
  $pdf->Write(0, $Display2[$i]["field"]);

}

$pdf->Output('File1.pdf', 'D');

Some of these forms are quite long, and it takes some time to figure out the coordinates. I've tried writing a JavaScript routine I can temporarily add to the page to figure the coordinates using a mouse, but I can't seem to find a reasonably workable calculation to convert the mouse coordinates to TCPDF's coordinates. The only alternative I can see is to create a grid and overlay it temporarily over the background form. There must be an easier way. What are TCPDF's units measured in? I can't seem to find that information.

[cpw]

What are TCPDF's units measured in?

The unit is set in TCPDF's constructor as the second argument, valid units are

• pt (point)
• mm (millimeter)
• cm (centimeter)
• in (inch)

The default value (if you don't provide one) is millimeters, also see
http://www.tcpdf.org/doc/code/classTCPDF.html#a134232ae3ad1ec186ed45046f94b7755