I downloaded some DEMs from the USGS with a resolution of 1/3 degrees for different areas in the US.

Now I am looking at the files and saw that all DEMs have the same number of columns and rows.

I measured the dimensions of the area covered by different DEMs and found that all have a similar length in N-S direction, but that their dimension in W-E direction varies a lot, being larger towards the south.

So now I am a bit confused. I know that the length of one degree longitude is larger near the equator than at the poles, so if my DEMs have a resolution of 1/3 degree I thought the number of columns in the DEMs for areas located in the north would be smaller than for areas located towards the south, but this is not the case.

On the other side, if DEMs at the north and south have the same number of columns, but cover areas of different dimensions, then the resolution must be different in both cases ...

Can someone help?

  • You must be looking at the DEM in a projected coordinate system. Rasters contain rectangular pixels - mostly rasters have square pixels. The geographic resolution and area covered is the same but when projected is quite different. – Michael Stimson Nov 6 '14 at 3:00
  • Yes, I am using a projected coordinate system. But I still do not understand how the area of a degree longitude has the same area in the north than in the south ... The "longitude lines" are much closer to one another in the north – Iced_ Nov 6 '14 at 3:03
  • A degree is a degree at the poles or at the equator, it's 1/360 of the circle, likewise a 1/3 by 1/3 degree cell is the same geographic size but the projected size differs greatly. There are cool interactive sites that show the distortions but I can't find one right now. – Michael Stimson Nov 6 '14 at 3:08
  • This link might help show how areas distort gmaps-samples.googlecode.com/svn/trunk/poly/puzzledrag.html and it's kind of fun. – Michael Stimson Nov 6 '14 at 3:25
  • It's a nice site! – Iced_ Nov 6 '14 at 3:46

The resolution is defined as 1/3 degree, that means three cells are one degree high or wide. Near the equator, the cells are square, but next to the poles they get to long rectangles, ending in a triangle at the pole itself.

If you want a resolution in real meters on the ground, you have to reproject the data to a projected CRS. Then you will get the rectangular size ratio you expected.

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