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Archive for August, 2007

Satellite Images for Global Climate Change

Friday, August 24th, 2007

Climate change refers to any significant change in measures of climate (such as temperature, precipitation, or wind) lasting for an extended period (decades or longer). As though much of its history, the Earth’s climate is changing right now and it is getting warmer. Most of the warming in recent decades is very likely the result of human activities. The National Oceanic and Space Administration’s (NASA) Surface Temperature Analysis indicate the average temperature of the Earth’s surface has increased by about 1.2 to 1.4 degrees Fahrenheit since 1900. Other aspects of the climate are also changing such as precipitation patterns and storms.

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Urban Sprawl – QuickBird Images
(Left Image is Before and Right Image is After )

Digital Elevation ModelQuickBird

Copyright © 2007 DigitalGlobe. All Rights Reserved

During the past century humans have substantially added to the amount of greenhouse gases in the atmosphere by burning fossil fuels such as coal, natural gas, oil and gasoline to power our cars, factories, utilities and appliances. The added gases, primarily carbon dioxide and methane are enhancing the natural greenhouse effect and likely contributing to an increase in global average temperature and related climate changes.

Land Cover and Land Use Change

In addition to changes in the atmosphere’s composition, changes in the land surface can have important effects on climate. For example, a change in land use and cover can effect temperature by changing how much solar radiation the land reflects and absorbs. Processes such as deforestation, reforestation, desertification and urbanization often contribute to changes (including temperature, wind and precipitation) in the places they occur. These effects may be significant regionally, but reduced when averaged over the entire globe.

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Color Infrared – Green Vegetation – Soil Index

The extent of these changes has prompted concern about the possible effects on the global physical, chemical and biological systems. Large scale changes in land use at rates unprecedented in human history are provoking considerable concern. Land use change is frequently accompanied by alterations or changes in land cover, which may possibly contribute to subsequent environmental change. Evaluation of static attributes (types, amount and arrangement) and the dynamic attributes (types and rates of change) on satellite image data may allow the types of change to be regionalized and the proximate sources of change to be identified or inferred. This information, combined with results of case studies or surveys, can provide helpful input to informed evaluations of interactions among the various driving forces.

Satellite Image data is expected to contribute to a wide array of global change-related application areas for vegetation and ecosystem dynamics, hazard monitoring, geology and soil analysis, land surface climatology, hydrology, land cover change, and the generation of orthorectified digital elevation models (DEMs).

Changes in land cover and land use can also effect the amount of carbon dioxide taken up (or sequestered) or released by the land surface. For more information, visit EPA’s Carbon Sequestration in Agriculture and Forestry site.

For more information on the state of knowledge of land use change on climate, see the Climate Change Science Program’s Strategic Plan’s Chapter on Land Use/Land Cover Change

What’s Known

Scientists know with virtual certainty that:

Human activities are changing the composition of the Earth’s atmosphere. Increasing levels of greenhouse gases like carbon dioxide (CO2) in the atmosphere since pre-industrial times are well documented and understood.

The atmosphere buildup of CO2 and other greenhouse gases is largely the result of human activities such as the burning of fossil fuels.

An “unequivocal” warming trend of about 1.0 to 1.7 degrees Fahrenheit occurred from 1906-2005. Warming occurred in both the Northern and Southern Hemispheres and over the oceans.

To View a Tour of the Cryosphere

To view a video of Ice Sheets Melting in Nares Strait between Greenland and Canada from 20th of January to 24th March 2007.

Video Credit – ESA – European Space Agency.


EPA – Environmental Protection Agency


Stereo Satellite Imagery and Digital Elevation Models (DEM) – Mapping the “Bare Earth”

Friday, August 10th, 2007

What is a DEM?

Digital Elevation Models are data files that contain the Mean Sea Level (MSL) elevation of the terrain over a specified area, usually at a fixed grid interval over the “Bare Earth”. The intervals between each of the grid points will always be referenced to some horizontal geodetic reference system and regional vertical datum. This is usually either latitude-longitude or UTM (Universal Transverse Mercator) coordinate systems and MSL elevation.

Within the same project area, a Digital Surface Model (DSM) represents the MSL elevations of the reflective surfaces of trees, buildings, and other features elevated above the “Bare Earth”.


3m Digital Surface Model (DSM) from Stereo IKONOS Satellite Image Data

The closer together the grid points are located, the more detailed the information will be in the file. The details of the peaks and valleys in the terrain will be better modeled with small grid spacing than when the grid intervals are very large. Elevations other than at the specific grid point locations are not contained in the file. As a result peak points and valley points not coincident with the grid will not be recorded in the file.

3D Satellite Image of Mendefera, Eritrea, Africa Southern Sahara, Tunisia, Africa
Mendefera, Eritrea, Africa Southern Sahara, Tunisia, Africa

IKONOS Satellite Sensor – DEM

Copyright © 2007 GeoEye All Rights Reserved.

The term Digital Elevation Model (DEM) has several meanings and is not always understood correctly or misinterpreted due to the surface it represents or geographic location the DEM data is being used. As used in most of the western hemisphere a DEM has “Bare Earth” z-values at regularly spaced intervals in x (Easting’s) and y (Northing’s). In some countries the x and y orientation are opposite from the general use with “x” being Northing and “y” Easting. Because of many different horizontal and vertical geodetic reference systems available around the world the necessity to clearly define the 3D geodetic reference system to be utilized is critical to a projects success to obtain the mapping standards defined.

DEM’s are utilized in support of the pre-planning of area developments for industries in Oil and Gas Exploration, Engineering and Construction, Environmental, Forestry, Geology and Agriculture applications.

There is a variety of DEM source data available for developed areas and the suitability of this available data is depending on the project specifications. In remote regions around the World, were little or no source Data is available, the DEM can be produced by automatic DEM extraction from stereo satellite scenes, from Satellite sensors such as IKONOS (2-5m resolution), SPOT-5 (5-10m res.) and ASTER (15-25m res.). The DEM can also be provided from Stereo Digital Aerial Photography at various resolutions, depending on the quality and scale of the Aerial Photography.

To represent the earth’s surfaces and bathymetry/hydrographic features in a 2D or 3D Geographic Information Systems (GIS) or CAD computer environment relative to existing geodetic horizontal and vertical datum’s the reference surfaces have to be clearly defined.


For practical purpose this “Bare Earth” DEM is generally synonymous with a Digital Terrain Model (DTM).Quality DEM products are measured by how accurate the elevation is at each pixel and how accurately the morphology is presented. Several factors are important for quality of DEM-derived products:

Terrain roughness

Sampling density (elevation data collection method)

Grid resolution or pixel size

Interpolation algorithm

Vertical resolution

Terrain analysis algorithm

Common uses of DEMs include:

Extracting terrain parameters

Modeling water flow or mass movement (for example, landslides)

Creation of relief maps

Rendering of 3D visualizations

Creation of physical models (including raised-relief maps)

Rectification of aerial photography or satellite imagery

Reduction (terrain correction) of gravity measurements (gravimetry, physical geodesy)

Terrain analysis in geomorphology and physical geography

For more examples of DEMs, please see our DEM gallery.