Raster GIS Lab: Map Algebra
Download lidar data for the North Carolina
coast near Duck. This will be a 160 MB download with a series of grids.
Go to the NOAA Coastal Lidar site,
and see if any newer Lidar data covers this area. If so, download it using
the graphical download option.
Find when it was collected in the metadata, and create a grid to match the
other grids. Lidar point clouds can have high (say a bird that caused a return
above the ground) or low values. You can
use percentiles when you display the grids to eliminate a few random points.
TIGER EDGES (under Features, all lines) file for Dare County,
NC, which includes this area.
Deliverables, in a single Word document:
- Are these maps DSMs or DTMs? Which should you create from the new lidar to best compare to them,
the ceiling or the floor?
- Consider 5 maps of the area around the Duck FRF pier with the
2008_NOAA_IOCM to discuss the following:
- What is the difference between creating a derivative grid for slope
and reflectance, versus symbolizing the DEM differently?
- Which maps (elevation or reflectance) have the most missing data, and why is this the case?
Show a figure(s) to support your answer.
- How well does the TIGER line data overlay the LIDAR? Which do
you think is more accurate (Lidar or TIGER), and why?
- Consider just the 1997 and 2008 DEMs, and a difference map between the
- There is an interesting pattern of change recorded around
N36.1730750° W75.7481630° (4 large rectangular regions about 50 m long
each). Do you think this is real, or an artefact in the data?
- How can you recognize new houses, destroyed houses, and moved houses
along this stretch of coast (include figures to back up your analysis).
- Is the 1997 DEM better or worse for long term comparisons with 2008
than the 1996 or 1998 data sets? Why?
- Do any of the later map show any changes from the 2008 survey?
Which of those appears to have the best quality data?
- Consider all of the maps
- What do the statistical maps from the DEMs (envelope and
standard deviation) tell you
about coastal change during this decade? How do they
compare with the simple difference map from 1997 to 2008?
- Based on what you see overall, does this area appear to be eroding or
You will create a number of new grids. You can save these to reopen
later (name them carefully, and remember where you saved them), or just recreate
them as needed.
New DEM and Open just the 2008_NOAA_IOCM data set.
New DEM. Close
everything, and open just the 2008 and 1997 DEMs.
- Create a Difference between the two
grids. The order in the operation is important,
because you want to differentiate erosion and deposition in this 10 year
period. Do you want positive to be erosion or deposition?
- Export this map to Google Earth, to see what the imagery says about
where the differences are.
Make a new grid with the data from the NOAA web site.
- Open the 2008 DEM.
LIDAR Point clouds to open the new
- Point cloud ceiling/floor
grids. Decide which best matches the other datasets.
- Interpolate across holes.
If there are holes in the data, you can interpolate across them. This
will not appear to fill a large fraction of the holes, but will fill a lot
of the holes in the portion of the DEM with data.
Open all the DEMs. Insure that you only have the DEMs open, and not any
of the derived grids.
You might want to look at this area in Google Earth, or try the
WMS map overlays to help with your
options on the Right
click on the map popup menu, which can show the location in Google
Earth, or USGS TNM viewer high resolution imagery.
We will not look at the Crab data this year, but you could look at it, and it
is an interesting data set showing changes in the nearshore environment over
Download the beach survey data
for the year assigned to you. From the surveys in this file,
take the closest one to the Lidar survey
grid for your year.
Coastal survey time series
Last revision 4/29/2017