Hi,
The Utm projection you are using is in meter so its easy to make square.
The MODIS tiff is probably in wgs84 so that would be in latitude and
longitude. In wgs84, the square will become a trapezium specially when
working over long distances or close to the pole.
Make sure "projection on the fly" is activated and open both layers.
Change the project projection to UTM and you will have a square. If your
raster looks distorted, you will need to reproject the vector layer (UTM) to
your raster projection...
That is easy to do but first, you need to add vertex to the square or else you
only have 4 corner points to work with.
You need to "complexify" the polygon. Look in the vector tools. Add some
point (like a 1000) then reproject the vector UTM file to the MODIS
projection. The vector should stop disappearing but it will be
rectangular in shape.
If you want a square in wgs84 but need to draw it in UTM coordinates, you will
need to figure out how many meters you have in one arc-second longitude and how
many meter you have in one arc-second latitude for that position in
space. You will then draw a rectangle in UTM that will look square in
WGS84.
Nicolas Cadieux M.Sc.
Les Entreprises Archéotec inc. 
8548, rue Saint-Denis Montréal H2P 2H2
Téléphone: 514.381.5112  Fax: 514.381.4995
www.archeotec.ca
On Apr 14, 2015 1:13 AM, "Don McNeil [via OSGeo.org]"
<[email protected]> wrote:
Dear QGIS Gurus: My problem is that, having already created vector
layers for a region in separate sub-windows of QGIS, when I create an
additional layer for a raster image by opening a MODIS TIFF file for the same
region, it appears as a trapezium rather than a rectangle, and when I try to
convert it to a rectangle by specifying the UTM location, the vector images
disappear. So I can't show both the squared raster image and the vector
images together. I emailed a friend who uses other GIS software (including
ArcGIS, ERDAS and IDRISI) and he suggested that I might need a process of
rectification for adjusting the shape and position of the image to the real
position instead of just changing the coordination. If so, does Quantum GIS
have a function to do that? kind regards....Don
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