This week in the lab we worked with scale and resolution. The lab focused on differences between line and polygon features in the first half and raster DEMs in the second portion described below.
To compare the two DEMs I focused on slope, aspect, and overall elevation.
First the slopes of each DEM were mapped and the averages compared. Then for
further analysis the slope maps were combined to display the slope difference between
the two DEMs. Second the aspect of each DEM was mapped and compared to analyze
the slope direction and general direction the terrain was facing. This analysis
gave more of a horizontal representation of the differences rather than
strictly elevation differences. Finally the overall elevation was compared for
the two DEMs. The standard statistics were calculated and compared to determine
the data represented.
The results clearly show that there is a difference between the SRTM
and LiDAR data. Both of the DEMs were analyzed at 90m resolution. The slope of
the separate DEMs seemed to follow the terrain feature of the area which was a
drainage. The LiDAR model had a steeper average slope which revealed itself in
the higher elevations of the terrain feature. The slope difference between the
models is also shown in the visual. The areas where the slope is the steepest
between the valleys and ridges tended to have a larger difference while
relatively flat areas had a smaller difference.
The aspect maps of the DEMs at 90m resolution were fairly similar. The
aspect analysis was meant to look at the models horizontal of directional
facing representations rather than elevation. The results show that even though
he models a very close match there are still some areas that have different
aspects. This shows that the slopes differed between the certain areas enough
to cause the software to display a different facing direction for the certain
area. The differences are slight and nowhere near opposite direction, but if an
accurate representation is vital then consideration should be taken in which
model to use for a study.
Finally the elevation of the two models differs. In the table the
results show that the LiDAR model has a lower average elevation along with a
higher maximum and lower minimum elevation. To me these results show the LiDAR
model as more complete. In the elevation comparison map the areas of largest
elevation difference follow the trend in the slope difference map. They are in
the specific areas of the terrain feature where the valley is transitioning to
a ridge and has a steeper slope. The one interesting result is that the large
differences in elevations are only seen on the southern side of the drainage in
areas with steep slopes. The norther side of the terrain feature shows smaller
differences between the two elevation models. This could be due to the location
of the sensor platforms when the separate model’s data was collected.
The creation of the LiDAR and SRTM both involve air/space borne
sensors. LiDAR can also be collected with ground based sensors, but larger
areas tend to be collected on using airborne sensors. These two sensors are
both collecting elevation data, but one is tens to hundreds of miles in the
air, while the other is less than a mile above the ground. The fact that the
SRTM data is at 90m is impressive considering how high up in space the sensor
is located. Overall I would have to say that the LiDAR model is a more accurate
representation of the drainage feature in all perameters due to the fact that
the sensor is collecting data far closer to the source, the model was derived
from a higher resolution product, and the LiDAR sensor was probably built for
high resolution large scale ground mapping.
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