I have been using height maps (and in particular, LiDAR data) to quickly design and test out ideas. However, though containing real world information, I've never tried to extract it in a precise or real world manner. So today's task was to pull out a contour map (the maps with the topographic lines, you know what I'm talking about) from the height maps I've been using. 

My amateur clumsiness fell into two hurdles;

1. Getting the correct coordinate reference system (CRS)
2. Data order

Both solutions are simple ...after you find the right information! So for the first, I've been using data from the environment agency in the UK (last year they made all that data free to download). Those maps use the reference system of the british national grid, ESPG 27700. So it's crucial you set the CRS accordingly.

You can then drape it across openstreetmaps or google;

Okay, so the data fits across another map. The CRS is aligned, the gods are pleased. But I bumped into my second mistake, which is equally as amateur. Upon opening the elevation table in QGIS I see this


The figures on the right, under ELEV, were supposedly the elevation height but it didn't make sense, they're all over the place and I couldn't work out what units they were in.

Well, first thing's first; they're not in order of elevation! Click the ELEV tab to order it correctly;


Blam. Now it's in order and the increments are in a 1000. But a thousand what? I thought the units were in metres, because the example in the tutorial was in metres, but nothing in england, nor earth is 360 000 metres tall. So Google Earth came to the rescue, the altitude cross referenced to 360 metres. Phew, this makes much more sense. Living in the UK, we use a dual system of imperial and metric. On paper, that seems like a cool thing, like a bilingual country. But it's contextual, we use imperial in one realm (like the speed of your car, altitude of a plane) and metric for another (product design). Sometimes they cross over (carpentry). At times of confusion, I find myself switching bases in a hope and hit manner, fingers crossed I land at something that makes sense. I failed. Today's lesson, always use google. Never think for yourself.

So I've been working in millimetres. Millimetres, what an insane world. Topographic data in millimetres. And what's painful, it says so on the tin;



One day, I will put up a complete tutorial on how to create contour maps from UK LiDAR data. I think it'll be useful. I make these newbie mistakes so you don't have to.