Discussion: GeoTIFF unproperly scaled

So I got the GeoTIFF loaded in for a mapping project.

But it is scaled wrong.

I import it, and then I draw a purple line on an area of the aerial photograph that should be 50 meters long. OCAD measures it to be 150 meters.

Having trouble with the GDAL documentation figuring a command line to set things right.

What version of OCAD? What scale of map are you trying to create?

Is it possible the 3:1 is not quite exactly 3:1? 1 foot is 0.3048m (except for survey feet).

Check your aerial metadata to see if it's in feet, and hopefully, exactly what projection it is in.

I use QGIS, which is sort of a front-end for GDAL. Typically, I Raster-Miscellaneous-Merge (?) to join 2-4 photos to cover a park using the original projection, then Raster-Projection-Change Projection (?) to gdal-warp to the new projection, then Raster-Extract(?)-Clipper to clip to the final park size (assuming a small-ish park).

I use "bilinear" instead of "near" during the change projection step.

You can use Raster-Projection-Extract Projection to create a wld file which you can open in notepad. Two of the entries in that little file are the size in meters of each pixel, two are the x-y / lat-long / of the upper left pixel, and two are the skew angle. So for 1ft aerials, the pixel size will be 0.3048m (approximately if they use survey feet), and for six-inch aerials it would be 0.1524 meters per pixel.

Actually, I might be a little confused here. It might be that some world files assume the "units" in "units per pixel" are defined by the projection that is set in the software. So if you've got a projection that uses feet, then it might show "1" in the world file meaning 1 foot per pixel rather than 1m per pixel.

Edit:

I'm fairly certain the units are defined by the projection that is selected. When you get aerials and world files from Pictometry, they're in WGS84 lat-long format, and the units per pixel has many zeros after the decimal point, indicating the units are degrees, not meters.

"units are defined by the projection that is selected"

Correct. :)

Also, OCAD assumes all units are metres. If your projection is in feet then you would end up with an image about 3x too big.

That's just unproper!

I think you're projecting.

OCAD11, trying to make a 1:15000 scale.

I'm suspecting that it is a feet / meters issue.

I'll verify that, and thanks, I'll play around with QGIS.

There are a lot of "quick and dirty" scale factor things you can do to get it in a form that will let you draw, but don't do them. Do the projection change so you preserve the georeferencing. It's hard at first, but it makes life easier later.

QGIS is really nice for this because you can verify that it loads in the right place using the "Web, OpenLayers plugin" by adding layers (including some aerial layers) from Google, Bing, Apple, Mapquest, or OpenStreetMap. If you rearrange the layers, the upper listed layers are on top, but there are also transparency settings. Sometimes they don't load well, and you have to zoom in and out or pan around to encourage it to load.

I like to use Google Earth to verify my projection, scale and UTM zone:

When I can export a kmz and it loads in the right spot I know everything is OK.