ASTER DEM Differencing and Image Analysis - Appendix
DEM differencing and image analysis was also conducted using The Advanced Spaceborne Thermal Emission and
Reflection Radiometer (ASTER) data for Santiaguito.
ASTER has a high spatial and spectral resolution with 4 VNIR (15m), 6
SWIR (30m), and 5 TIR (90m) bands. Information on ASTER can be found here.
ASTER is unique in that the satellite has a backward looking band
(3B) which allows for stereoscopic viewing capabilities for creating
DEMs. The ASTER DEM analysis resulted in
similar elevation changes both inside and outside of the volcanic zone, so they were not used in this research.
Many necessary insights were gained through this analysis which
is described below. In addition, I will discuss how ASTER DEMs
were created and analyzed in this study.
ASTER analysis was conducted for Santiaguito and five images were found that were cloud free. Of
those five, three were nearly the same date: Jan 4, Jan. 20, and Feb 12, 2005, while the others were obtained on Dec. 21, 2002 and March 13, 2004. The
December and March images had a plume, but much of the plume was north
of the dome and was masked out. Again, due to the lack of GCP’s, relative
DEMs were created by the ASTER 3N(Nadir) and 3B(Backward-looking) bands using
an automated stereo-correlation method, creating 15 meter resolution DEMs, for Dec.
21, 2002, March 13, 2004 and Jan. 4, 2005 using SILCAST
1.0 developed by Sensor Information Laboratory Corp (SILC).
SILCAST 1.0 was available through the Michigan Tech Research
Institute and the DEMs were created by the staff and sent to
MTU. The DEMs created from SILCAST, were relative, meaning no ground control points were used.
Note: ENVI has a DEM
extraction tool which was initially used for this study. This
tool takes the user through the process of choosing common points on
the backward (3B) and forward (3N) looking bands and then creates a
DEM. This tool created poor DEMs and was not used in this study.
In addition I tried using ERDAS Imagine to create DEMs using stereo
pairs and poor DEMs also resulted. The DEMs created through SILCAST
were the best quality of ASTER DEMs I have seen. The quality of ASTER
DEMs is going to be poor when compared to aerial derived methods, but
in this case I wanted to analyze changes from 2000-present and these
were the only DEMs available. The SILCAST processing used only the
entire image was used to create the DEM, where masked regions were used in ENVI.
Once the DEMs were created, the Jan. 4, 2005 and Dec. 21, 2002 DEMs were used for DEM differencing.
The images are shown below with the 3N band for each date and the
corresponding hillshade (derived from the DEM). The difference in
quality to the aerial derived DEMs is clear, as shown here
(hillshade of Santiaguito). The ASTER derived DEMs result in
reduced resolution and appear to be grainy. I used the DEM differencing procedures to create the elevation change which is overalyed on the Jan. 4, 2005 image.
The area which is masked out contains the plume from the Dec.
2002 image. Although some changes are noticeable within the dome
region, many of these changes are similar to the surrounding areas.
Ultimately, the
volume change analysis between ASTER DEMs for 2002, 2004, and 2005 were
not significant
due to regions outside of the volcanic zone having similar elevation
changes to regions within
the volcanic zone. No significant changes were found within the
volcanic zone. This is not surprising due to the short temporal scale
of DEMS (2002 through 2005) and the 15 m resolution of the DEMs.
Significant activity would have to occur during this time period
for elevation changes to be present.
In
addition to the above analysis, the three ASTER generated DEMs were analyzed
with the aerial derived 2001 DEM to obtain elevation and volume change. The
three ASTER DEMs were shifted in the x and y directions to align with the 2001
DEM using the UTM Zone 15 WGS 1986 coordinate system. Elevation change for the 2001 DEM and the
three ASTER DEMs are shown below, revealing changes on the Caliente dome.
Also shown below are the elevation differences outside of the Caliente region:
The clouds and plumes are masked out in each image. The elevation
changes inside and outside of the volcanic regions were very similar.
The mean elevation was lower for regions outside, but not
significantly. Due to similar elevation changes inside and outside the
volcanic area and the higher error for ASTER DEMs, vertical error of 10
-30 meters
(Lang et al. 1999) the ASTER DEMs were not used for this analysis. For 2001-2005 a total volume change on Caliente was 0.19 km3
(1.49 m3 s-1), with a possible volume error estimate of 0.53 – 1.6 km3. Using the error
range for ASTER DEMs, errors were much larger than the calculated
volume change and were therefore not used.
Although the
ASTER DEM analysis was not successful, ASTER images were still analyzed
and visible changes at the volcano are shown below:
The images acquired
on Dec. 21, 2002, March 13, 2004, and February 12, 2005 are shown above with the visible, near-IR, and thermal IR bands. Changes on the
Caliente dome are seen within the highlighted boxes. Activity during this period was consistent
with previous activity including explosions, ash emissions, blocky lava flows,
pyroclastic flows, and lahars (BGVN 2003, 2004, 2005). From the second row of imagery, deposition
from a blocky lava flow is shown, while row 3 shows a change in channel
morphology. Lastly the thermal IR band is shown, revealing activity in each of the images.
Conclusions
Creating ASTER DEMs are going to yield somewhat grainy results,
when compared with those from aerial derived methods. If ground
control points exist for a study site the ASTER DEMs should greatly
improve and have decreased error. Unfortunately for
this study, there were few images available over a large temporal scale
and no ground control points were available. Ultimately ASTER imagery
was used to chronicle changes happening at the volcano, which can help
with further analysis.
I have created many references related to ASTER and ASTER DEMs which can be found here.
Copyright information for ASTER images is located under the JPL Image Use Policy located here.