Interpolation is a commonly used GIS technique to create continuous surface
from discrete points. A lot of real world phenomena are continuous -
elevations, soils, temperatures etc. If we wanted to model these surfaces for
analysis, it is impossible to take measurements throughout the surface. Hence,
the field measurements are taken at various points along the surface and the
intermediate values are inferred by a process called ‘interpolation’. In QGIS,
interpolation is achieved using the built-in Interpolation plugin
.
We will take field depth measurements for a Lake Arlington in Texas and create an elevation relief map and contours from these measurements.
Texas Water Development Board provides the shapefiles for completed lake surveys.
Download the 2007-12 survey shapefiles for Lake Arlington.
For convenience, you can directly download the sample data used in this tutorial from link below.
Data Sources: [TWDB]
Shapefiles.zip
file and select it. Click
Open.Shift
key
and select Arlington_Soundings_2007_stpl83.shp
and
Boundary2004_550_stpl83.shp
layers. Click OK.Boundary2004_550_stpl83
layer represents the boundary of the lake. Un-check the box next to it in
the Table of Contents.Arlington_Soundings_2007_stpl83
. Though the data looks like lines, it is
a series of points that are very close.ELEVATION
attribute contains the
depth of the lake at the location. As our task is to create a depth profile
and elevation contours, we will use this values as input for the
interpolation.Interpolation plugin
enabled. See
Using Plugins for how to enable plugins. Once enabled, go to
.Arlington_Soundings_2007_stpl83
as the Vector layers in the
Input panel. Select ELEVATION
as the
Interpolation attribute. Click Add. Change the
Cellsize X and Cellsize Y values to 5
. This
value is the size of each pixel in the output grid. Since our source data is
in a projected CRS with Feet-US as units, based on our selection, the
grid size will be 5 feet. Click on the … button next to
Output file and name the output file as elevation_tin.tif
.
CLick OK.Note
Interpolation results can vary significantly based on the method and parameters you choose. QGIS interpolation supports Triagulated Irregular Network (TIN) and Inverse Distance Weighting (IDW) methods for interpolation. TIN method is commonly used for elevation data whereas IDW method is used for interpolating other types of data such as mineral concentrations, populations etc. See the Spatial Analysis module of the QGIS documentation for more details.
elevation_tin
loaded in QGIS. Right-click
the layer and select Zoom to layer.elevation_tin_clipped.tif
. Select the
Cliiped mode as Mask layer. Select
Boundary2004_550_stpl83
as the Mask layer`. Click
OK.elevation_tin_clipped
will be loaded in QGIS. We will now
style this layer to show the difference in elevations. Note the min and max
elevation values from the elevation_tin
layer. Right-click the
elevation_tin_clipped
layer and select Properties.Singleband pseudocolor
. In the Generate new color map
panel, select Spectral
color ramp. As we want to create a depth-map as
opposed to a height-map, check the Invert box. This will assign
blues to deep areas and reds to shallow areas. Click Classify.0
as the Additional no
data value. Click OK.contours
as the
Output file for contour lines. We will generate contour lines
at 5ft intervals, so enter 5.00
as the Interval between
contour lines. Check the Attribute name box. Click
OK.contours
layer once the processing
is finished. Right-click the layer and select Properties.ELEV
as the field. Select Curved
as the
Placement type and click OK.This work is licensed under a Creative Commons Attribution 4.0 International License