How can I get depth data from Kinect?

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Hi guys, I'm quite new to ROS. I'm using the Ubuntu 12.04 and openni driver.

What I've been trying to do is to use Kinect to detect whether a door is open or closed. My idea is to write a node to subscribe to camera/depth/image, and and can retrieve a distance at the center point in the image.

But when I use rostopic echo camera/depth/image I just get a lot of numbers like 0, 0, 65, 128, 36, 56, 77, 102, 140, 65. So does anyone know what should I do with these numbers, or are there other ways to get depth data?

Firstly, thank Martin Günther's answer, it really help me a lot. And I already solve the problem by using OpenCV. Here is part of my code, it's very easy. If some beginners cannot understand that hackish solution, maybe you should try this one:

#include <image_transport/image_transport.h>
#include <cv_bridge/cv_bridge.h>
#include <sensor_msgs/image_encodings.h>
#include ..........

namespace enc = sensor_msgs::image_encodings;
void imageCallback(const sensor_msgs::ImageConstPtr& msg)
{
    cv_bridge::CvImagePtr cv_ptr;
    try
    {
      cv_ptr = cv_bridge::toCvCopy(msg, enc::TYPE_16UC1);//now cv_ptr is the matrix, do not forget "TYPE_" before "16UC1"
    }
    catch (cv_bridge::Exception& e)
    {
      ROS_ERROR("cv_bridge exception: %s", e.what());
      return;
    }

    int depth = cv_ptr->image.at<short int>(cv::Point(240,320));//you can change 240,320 to your interested pixel
    ROS_INFO("Depth: %d", depth);
}

int main(int argc, char **argv)
{.........}