Coordinates - Horizontal tilt sensor and coordinate transforms

(Posted by ehackett on Nov. 20 2007)

I have a Vector with a horizontal tilt sensor.  I have collected data in ENU and want to convert it back to xyz.  Can I use the Matlab script for coordinate transformations with the horizontal tilt sensor, and if not, how do I need to modify it?

Also, when the data is collected in xyz with the horizontal tilt sensor: the 'X' data is the vertical component, 'Z' data is along the instrument axis (horizontal in this case), and 'Y' data is 3rd orthogonal component, is this correct?  Because in the data, I would expect the vertical component to be the smallest but the 'Z' data is showing the smallest component, which is inconsistent with my expectations b/c the 'Z' data is actually in the horizontal plane (as far as I understand).  Are the X and Z data switched prior to being output to the data file in Vector's with a horizontal tilt sensor?

(Posted by Atle Lohrmann on Nov. 23 2007)

Hi there,

have you checked this drawing?  Vector cable probe

(Posted by ehackett on Nov. 29 2007)

I hadn't seen this, but it does not help me too much.  My Vector is not a cabled version, it is rigid.  The explanation of the cartesian coordinates is consistent with my current understanding that the output 'Z' velocity is acutally the X velocity, but it is not still not clear to me whether I am able to use the same coordinate transformation matrices with the horizontal tilt sensor as is used with the vertical tilt sensors.

(Posted by Atle Lohrmann on Dec. 03 2007)

Hello

The coordinate transform is independent of the orientation of the tilt sensor. So, you can use the same math whether the tilt sensor is vertical or horizontal.

Best regards, Atle Lohrmann

(Posted by ehackett on Dec. 06 2007)

Thanks!  

This is still a bit confusing.  I used a vector with a horizontal tilt sensor, but I needed to mount it upward looking.  (Beggars can't be choosers!)  Therefore my vertical axis was along the body of the instrument which the drawings define as Z.  My Z velocities (easily identifiable as the small ones) were reported in the X data.  This seems to indicate that the x and z axes are indeed switched.

By the way, the vector documentation shows Z positive towards the instrument where the drawing linked above shows z pointing away from the sensor head.  Is this an inconsistency?  Are they both correct?

Hello,

I am not sure if I understand your question correctly, but the drawing shows how to mount the probe in order to align it correctly to the housing if you select to use ENU coordinates as the instrument configuration. If you select to use XYZ coordinates the attachment shows how the coordinate system is defines. The XYZ coordinate system is always defined according to the probe like it is shown in the linked drawing. You may also freely place the probe any way you like if you are using XYZ coordinates but if you are using ENU coordinates you have to make sure that you align it correctly to the instrument housing. I hope this answers your question.

Regarding the sign of the Z axis the linked drawing is correct whereas there is unfortunately an error in the August 2005 version of the Vector manual.

Best regards,
Sven Nylund

Vector cable probe

Let me try to clarify my situation a bit.  I borrowed a vector with a horizontal tilt sensor.  The probe is attached directly to the instrument (no cable).  I deployed the instrument with the probe (as well as the rest of the instrument) looking upward.  I configured the instrument to report in XYZ coordinates.  While inspecting the raw data it is easy to identify the true vertical (assuming the instrument is truly vertical) axis by scale since the data are an order of magnitude smaller than the horizontal axes.  The data stream identified in the header file (*.hdr) as being for X values (the first of the three velocity values in *.dat) contains the data that are an order of magnitude less than the other two axes.

The prior paragraph contans facts.  What follows is speculation.  Since this instrument was intended to be mounted horizontally, if the X and Z axes were switched with respect to the drawing linked above then the X and Y axes would be horizontal axes defined by which direction the instrument is aimed (X direction) and Z would still be a vertical axis.  (This is presuming the instrument is mounted horizontally as intended, with zero pitch and roll.)  This is a "sensible" way for the axes to be oriented.  The question is: Is this true?  If so, this is an undocumented "feature" of the Vector when configured with the horizontal tilt sensor.

Does this clarify my question?

Tony

Dear Tony, sorry but I am afraid that the combination of OI in London, CMTC in Charleston and Easter vacation really slowed us down.

1) The coordinates systems we use are determined by the compass orientation and the need to get the correct transform from XYZ to ENU. As such, the XYZ coordinate systems are not arbitrary but determined by the question "What does the XYZ system have to look like if the compass/tilt sensor transform is to be correct?"

2) In case of the Vector with horizontal tilt, the X axis is in the direction of the canister (postive away from it), the Y axis is to the left in the horizontal plane if the instrument is lying down and the Z axis is up (right handed).  When you mount it vertical and upwards, X will point up and Y/Z will be in the horizontal plane.

Hope this is getting clearer.   Best regards, Atle Lohrmann

Hi Tony,

Sorry for the confusion, the drawing in the Vector manual is correct. It shows the XYZ coordinates for the standard system with fixed stem. This does not imply that the linked drawings in this thread is incorrect since it all comes down to the combination of compass orientation and the transformation matrix in the various configurations we use.

Best regards,
Sven Nylund