Getting hold of a good photo reference is normally only half the battle, after that you have to work out how to replicate it on your painting surface!

This is the reference image I used for this demonstration, a photo I took of the
Bombay Gin Distillery near Basingstoke. As you can see this is a complex reference image that contains perspective and buildings of different sizes, plus reflections, etc. Getting a good representation of this reference image is important and the process described below should help you with that.

I have a free image editor called "GIMP" that works very well to help with the sketching process.

GIMP is a very powerful image editor, some even say that it rivals Photoshop in what it can achieve, however the big difference is of course that GIMP is FREE to use. I have described below a sequence of actions that tell you where to go within the GIMP menu system and what to select at each stage. You should end up with an outlined image at the end as shown at the bottom of this article. Once you have your outline printed out then it is straightforward to put it beneath your paper and lightly trace the important lines to get perspective, relative size and alignment of key objects correctly.

You can make this transfer process even easier using tracedown paper or a light box or if you have neither of those things then make use of a window in the house! If you tape your printed image to a window the light from outside with act as your light box enabling you to be able to see the printed image beneath your watercolour paper.

The advantage of lightly tracing your image in this way is that it saves all of the rubbing out and trial and error associated with free hand sketching and this in turn saves disturbing your painting surface as little as possible.

This is the sequence of steps to follow:

Step 1: Having loaded in your photo reference image into GIMP the next to do is to go to the menu system and Select Menu Filters – Edge Detect – Edge and that will produce this effect. 

Here we have our outlines but they are shown in white so we need the reverse of this image.

Step 2: To invert this image select Colours from the menu and then Invert and you will get this effect which is a bit easier to see what is what. This is a colorised version of what we actually need. So one more simple step can be taken to recitify this and get what we are after!

Step 3: Now convert this image to a more black and white version for clarity. You do this by choosing Colourize from the Colour menu option and then in the popup dialog move the slider across to the left and position it half way down so that you end up with something that resembles this.

This is now a good clear outline to work with and makes it easy to pick out the important lines and points to transfer to your watercolour paper. This will ensure that you end up with clear guides for what goes where, it's proportion and realtionship to everything else in the drawing. How much you actually trace is entirely up to you when you take this approach. The main thing is that you will feel much more comfortable completing the drawing and ending up with a really good sketch ready for the all important painting process.