One of my closest friends and I were able to use a radio telescope to measure the 21cm spin-flip transition line of elementary hydrogen present throughout our galaxy. Looking at the Doppler shift tells you whether the material is moving towards or away from you.
This image shows the heat-map of the hydrogen in our galaxy, with the vertical axis representing the wavelength (centered on that 21cm line) and the horizontal the direction of the telescope.
By taking that data in combination with some assumptions about distance and speed, the density of the hydrogen can be converted into a rough map of our home galaxy.
More data and better assumptions would produce a more accurate result, but nonetheless the Sagittarius and Perseus spiral arms may be roughly seen.
