Path: csiph.com!x330-a1.tempe.blueboxinc.net!newsfeed.hal-mli.net!feeder1.hal-mli.net!border3.nntp.dca.giganews.com!Xl.tags.giganews.com!border1.nntp.dca.giganews.com!nntp.giganews.com!local2.nntp.dca.giganews.com!nntp.earthlink.com!news.earthlink.com.POSTED!not-for-mail
NNTP-Posting-Date: Thu, 31 Mar 2011 19:53:58 -0500
Date: Thu, 31 Mar 2011 17:53:56 -0700
From: Patricia Shanahan <pats@acm.org>
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.15) Gecko/20110303 Thunderbird/3.1.9
MIME-Version: 1.0
Newsgroups: comp.lang.java.programmer
Subject: Re: Android Sensor Coordinates
References: <in34ov$2ts$1@lust.ihug.co.nz>
In-Reply-To: <in34ov$2ts$1@lust.ihug.co.nz>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Message-ID: <Tu6dncwgoMi7vwjQnZ2dnUVZ_gmdnZ2d@earthlink.com>
Lines: 23
X-Usenet-Provider: http://www.giganews.com
NNTP-Posting-Host: 75.8.126.96
X-Trace: sv3-k08cCFCVGEtweXX/qM4yk2R5NtLryO2ps+egfpOA0l6eYkO6RCDSZv6bYfAoos3e8KfTW9ONJoebMS+!N0d7pHQ0i+WKck/W5gjLEy/Ya2ZEIcJDEyWHjlOfbyJKGAhHXS8mvUDnQuqyrdlyTvM6KZp/I97h!sZTcVwUa91NgvgP8poHdtuRMz0SjNdw6U2TTcytDO/Y=
X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers
X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly
X-Postfilter: 1.3.40
X-Original-Bytes: 2186
Xref: x330-a1.tempe.blueboxinc.net comp.lang.java.programmer:2683

On 3/31/2011 5:03 PM, Lawrence D'Oliveiro wrote:
> The diagram of the linear sensor coordinate system here
> <http://developer.android.com/reference/android/hardware/SensorEvent.html>
> seems to be wrong.
>
> I have a test program running on my HTC Desire (Android 2.2), showing me
> values from the accelerometer. As it lies horizontally, face-up on my desk,
> the Z value is positive (around 9.8, as you would expect). But that diagram
> says that positive Z is up, not down.

Maybe it is showing deviations from free-fall? The accelerometer can
presumably only measure forces that act differently on the case and the
insides.

As a thought experiment only, consider what would happen if the desk
were not there, and the device were in free-fall. It would move towards
the center of the earth, in the negative Z direction. The accelerometer
would briefly report zero.

The device's deviation from free-fall is an acceleration upwards, due to
the upwards force the desk exerts on the device.

Patricia