Getting the offset of a placed PDF within the container rectangle

Question

I need to get the offset of a placed PDF inside of the container rectangle. I have figured out that this information should be available in the transformationMatrix of the placed file. So I do something like:
var pdfTransform = pdf.transformValuesOf(CoordinateSpaces.pasteboardCoordinates)[0];
var horzTranslation = pdfTransform.horizontalTranslation;
var vertTranslation = pdfTransform.verticalTranslation;
but the coordinates I get back are just weird. The horizontal value seems reasonable, but the vertical is way off what I expect.
There may be some precondition to doing this that I'm missing. Running the pageitems/TransformValuesOf.jsx sample from the Scripting Guide also gives weird values. That sample creates a rectangle with a center point of [0, 0] and then retrieves the transform. I would expect the translation to be [0, 0] but I actually get [5, -415.95] (horz, vert).
In that sample it talks about ruler coordinates, so I tried setting document.viewPreferences.rulerOrigin to different values, but that does nothing.
I'm stumped. Can someone please give me a hint on what's going on here?

Answer 1

Thanks Peter.
Some more info: I basically want to get the coordinates of the objects in a placed file in the coordinate space of the containing document. As an extra twist, the placed file is a PDF created from an InDesign document. So, I first extract the coordinates from that InDesign file, then the coordinates for the document that contains the placed PDF, and finally I merge the coordinates of the two documents as if all the objects were contained in a single document. This workflow is then used outside of InDesign to draw frames around all the object boxes on a JPEG created from the document containing the placed PDF.
Anyhow, to answer my own original question, this seems to work thus far:
rectOffset = containingRect.resolve(AnchorPoint.TOP_LEFT_ANCHOR, CoordinateSpaces.PASTEBOARD_COORDINATES)[0];
pdfOffset = pdf.resolve(AnchorPoint.TOP_LEFT_ANCHOR, CoordinateSpaces.PASTEBOARD_COORDINATES)[0];
horzTranslation = rectOffset[0] - pdfOffset[0];
vertTranslation = rectOffset[1] - pdfOffset[1];
That gives the same values as I can see in InDesign when selecting the placed PDF. So far so good ...
BUT, that is not enough! When placing a PDF using the default option of Crop to Bounding Box, I sometimes get a bit of extra margin (different for different documents) that offsets the translation values. I have no idea where this margin comes from but I need to compensate for it.
Please help! And let me know if this description is unclear so I can try to explain better.