Working with DEM Models Qgis Quantum GIS

July 11th, 2013

Below you will find a few tips on working with DEM models in Qgis. Image taken from http://earthexplorer.usgs.gov/ of Big Island Hawaii

You download your GeoTiff file and it looks like this:

 

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To fix it right click on the layer>properties>Style Tab. Under contrast enhancement choose StretchtoMinMax

 

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It should look like this:

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Still not much but better. This is an image of The Big Island Hawaii . The lighter the color the higher to elevation and the darker the color the lower. Since this is an island the dark is water in this case

Let’s run a few raster analysis tools to learn more about it.

First let’s calculate slope. You can either express slope as a percent or as is. If you have issues when creating your raster analysis and it comes out looking much like a tv screen with static you need to convert the unit of the layer to the units of the map. If you map is in meters then it is 111120. If it is in feet 370400. It is input whenever you see this box here:

Calc

Slope as a percent:

slopeaspercent

Slope

 Slope

For this exercise I used a scale of 111120 to convert it to meters

Next we can add contours to the map:

Raster>Extraction>Contour

When choosing an interval be careful. It is going to generate a line shapefile that can be very large so just play around and see what work for your project and make sure to include it somewhere on the map or you can label the contour lines also.

I used 100 meter intervals to create this map. I also turned off the layers so it can be seen better

contour

These are just a few of the tools available in Qgis. Feel free to leave a comment requesting help or any other tutorials.

Free Courses

May 1st, 2013

Coursera is a website that has 100’s of courses from Intro to Algebra to History of Rock. The website is a collaboration between several universities around the world such as Penn State and University of Tokyo just to name a few.  All courses are free to take and come with a certificate of completion at the end. If you want something to add to your resume or just to learn a new skill this is something to look at.

 

www.coursera.org

Projection/ Coordinate Systems in Quantum GIS

April 23rd, 2013

Projection is very important when creating maps.  They can create distorted maps or any analysis of the information can be incorrect.

 

To set the projection for an entire project

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The box below will open up.

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‘On the fly projection’ in quantum means that the as the layers for the project are added they will be projected in whatever coordinate reference system you have chosen. This does not actually change the coordinate system of

the layer just how they are projected.

 

For this example on the fly is not selected. When this option is turned off the layers will project as is and you can also change the project units.

 

 

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When on the fly is enable these options will gray out and then you will be allowed to set the projection for the project.

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Keep in mind you can not change the units as these will be set by the CRS you choose.

 

To change the CRS of a layer it needs to be saved with a new CRS.

 

Right click on your layer>Save As

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You have three options for the CRS. You can either save it with the CRS it is in (Layer CRS)

You can set it to the project (Project CRS)

Or you can choose a new CRS from the list (Selected CRS)

 

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To see the CRS of your layer there are a few options.

Right click on the layer>Properties>Metadata Tab and then look under Layer Spatial Reference System.

 

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Or find where you have your layers saved

Right click on the .prj and choose open. If it prompts you to choose a program from the list/installed programs  select Notepad. This only needs to be done once.  Just be sure to check always use for this

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And it opens a notepad with the projection and CRS information.

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Print Composer in Quantum GIS

April 15th, 2013

Print Composer is the tool used by Quantum to ready maps for presentation. In it you can add elements such as a title, legend, scale bar and north arrow. When creating a map for presentation consider your audience. If the map is for others you work with maybe you don’t need to add the symbology for a road or well since it is already known by everyone. If this same map is to be posted online you need to make sure the audience can look at the map and understand it without too much difficulty. Here are just a few of the things available in print composer

File>New Print Composer or CTRL+P

 

When it opens you have a blank page.  Here is where you want to adjust your paper size. That will give you more area to work with when adding elements and if you change it later you may have to move the elements again.

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To add a map. Click on add a map at the top or Layout>add a map.  Then click on any corner and drag the box until you reach the size you want your map to be and then release.

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To edit the location of the entire map on the paper click this symbol. This will move not only the map but also the highlighted box. 0415_3 To move just the map in the space click this button 0415_4 .

0415_9 Adds a directional arrow depending on the direction of the mouse

0415_8 Adds either a triangle, rectangle or ellipse

0415_7 Adds a new scale bar

0415_6 Adds a legend

0415_5 Adds a label (text box)

When something is added to your map it becomes an item. Then you can choose item properties to change any attributes you want.

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In this example I have added a label. In the item properties dialogue box there is a heading called label and another below it called General Properties.  When an item is added all the editing is contained here. Some will have different headings.

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These are just some quick tips on print composer. What some of the buttons mean and where to go when you want to edit an item.

Creating a shapefile Layer in Quantum GIS

April 8th, 2013

In Quantum you can create either a point, line or polygon layer.

Layer>New>Shapefile Layer

Shapefile Layer

Then you can either choose to create a point, line or polygon layer. Before choosing think about the data you are using and what you are creating. If you are working with US Highways and are now creating a layer that just includes highways in Texas you want to choose line for the type of file you create.

For this example Line was chosen as the type of layer.

NewVector

Next choose a CRS or coordinate Reference System. You can either leave it as the default and then change it later (see below) or you can set it when you create the layer.

You’ll notice that the id field is automatically populated. You can delete this field if you don’t want it there. Select id and then click remove attribute.

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To add an attribute:

Under ‘New Attribute” type in the name select the Type  of data the field can hold. You have several options. One is text data this refers to letters or numbers. Then you can choose whole numbers. This refers to numbers such as 100, 300, 500, 10,15,1000. You can not input decimals into this field. Lastly there is decimal number. This includes numbers such as 12.15, 11235,2569843.

You can also choose the width and the precision. Width can be set with all fields. Whole, Text and Decimal. Width refers to the number of characters allowed in the field. This includes punctuation and spaces.  For whole numbers the maximum width is 10. Precision can be set when decimal is highlighted. Precision is the number of places allowed after the decimal. It is entered as zeros in the precision field.

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Once everything is set click on add attribute.

And it will appear in your list below. You can continue like this and add as many field as you like. They can be a mix of text, whole or decimal.

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Once you are done click ok. It is going to ask you to choose a location to save. When you choose a name to save it as it will give it that same name in the attributes table but you can always change that later.

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If the CRS was not set when the layer was created. You can set it now. Right click the layer and then go to set layer CRS.

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Then you can set the CRS.

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Lastly save the layer.

Right click>save as

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You have several options.

Choose the format . It defaults to ESRI shapefile.

Give it a name and choose where you want to save it.

Then you can set the CRS here also.

Layer will leave the CRS as the default. Either WGS if you made no changes when you created it. Or what it was set to when it prompted for the CRS originally. Project will set the CRS to what the project is, if that was set up. Lastly is selected where you can browse and set the CRS to what you want.

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Once your done click ok!