QGIS - Question

[gldnbrew]

Been using QGIS for about a month now using 1 meter LIDAR image tiles, and there is an item I am having trouble with.

When I load the raster images into Qgis there are two numbers that appear below them (Ex: 75.345 and 235.589). My question is, are these the min. - max elevations of the tile or are they some numerical value? The reason I ask, is I have three adjoining tiles with a wide creek in them, but the lower number (75.345) which I am assuming is the creek does not appear on the other two tiles. When I do a hillshade and then do a contour, assign the automatic color ramp, none of the adjoining tiles match in color, some colors not even close for the same contour. I have tried importing a custom color ramp of the min - max using all three tiles, but that is not working either. How do I correct this? Also, I am assuming that these numbers are in meters, is that correct.

Thanks,
Paul

 

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[Clay Diggins]

You need to merge the the DEM rasters before you process them Paul. The numbers you are seeing are only relative to each individual DEM. Once you merge the DEMs the gradient and contour functions will apply evenly across the entire merged DEM.

Depends on what projection your DEMs were saved as but most often the final numbers are in meters. And yes they do represent relative elevation.

 

[gldnbrew]

Clay,

Thanks for the response. I will do the tutorial as you suggested. I use Qgis for the primary purpose of laying out a relief terrain of a battle area, using layers to plot all the shot (by size, about 300 to date), and position of troops and their movements for a War of 1812 Battle I am researching. Fortunately there is a 1 meter DEM available for my county, however I do have one area I would like to have a higher resolution, which I understand is not available to the public.

A note for Relic Hunters

I also have discovered a secondary use for Qgis and I say a very valuable tool for relic hunters, in which a 1 meter DEM is great for finding the actual location of old homesteads, roads, and fences which appear on old maps, and which can not be seen in Google Earth. [Download the DEM for that location, bring into Qgis > go to raster (on tool bar) > to Terrain Analysis > hillshade. Once hillshade is in the layers panel you will be able to find old cellar holes and other features, some will be more obvious than others, for the unobvious look for shapes that do not appear in nature.]

Clay, thanks again for your knowledgeable help.

Paul

 

[Eu_citzen]

Curios, how's the progress with QGIS? I'm also a user of QGIS, quite new as well.