Global basemaps products - Overview
Large-sized sample - The entire U.S. West Coast including California (30m-resolution)
Two complimentary elevation data products
(1) Near-global SRTM elevation mosaic and shaded-relief basemaps;
(2) Global elevation mosaic resampled at 30m resolution and shaded-relief basemaps (higher latitudes filled by other elevation sources at closest resolutions)
Shuttle Radar Topography Mission (SRTM), flew on the Space Shuttle Endeavour launched on 11 February 2000, had successfully collected 3D measurements of the ~80% Earth's surface. The mission was a cooperative project between NASA, the National Geospatial-Intelligence Agency (NGA) of the U.S. Department of Defense, and the German and Italian space agencies, and was managed by NASA’s Jet Propulsion Laboratory (JPL) for NASA's Earth Science Enterprise, Washington, D.C.
The USGS freely distributes the latest SRTM Version 2 (also known as the "finished" version) data set, which is the result of substantial editing effort by NGA. It features the following:
- Well-defined water bodies (e.g., lakes, reservoirs
and major rivers) and coastlines;
- Absence of spikes and wells (single pixel errors); and
- A small percentage of elevation voids (gaps, holes or missing data) still present.
We advocate the importance and potential of this
public-domain data set, and have undertaken significant
post-processing to produce a seamless,
highest-resolution, near-global elevation mosaic and a
series of cartographically-designed, GIS-ready shaded
relief basemaps. This task is recently updated for
Here are a few important steps:
Step 1: Stitching 14,000+ separate SRTM tiles
together to make a seamless near-global mosaic
(data size: 432,000 X 139,200 pixels)
Step 2: Expanding to the worldwide coverage (data size: 432,000 X 216,000 pixels)
Elevation at higher latitudes can be filled by other global elevation sources at closest resolutions possible (as part of GeoSage's processing services), such as the recently released USGS's Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010). The low-resolution SRTM30 or GTOPO30 data is NOT used here. An overview is provided in the following figure.
Step 3: Filling gaps
A significant part of our post-processing is to fill some gaps in the original SRTM tiles. An illustration is shown below:
Step 4: Masking with land / ocean boundary
A series of shaded-relief basemaps are produced under
various colour schemes and hillshading settings.
Nine hillshading colour schemes are shown in this overview. Each basemap is with the image size of 432,000 X
216,000 pixels. While the size of these maps is large,
virtually all major GIS systems these days can display
large-sized JPEG2000 imagery effortlessly.
Step 5: Making shaded-relief basemaps (90m resolution)
Shaded-relief basemaps vividly illustrate both hillshading effects and topographic heights in either colour gradients or grey levels. Efforts have been made to highlight and better visualise geological and geomorphologic features such as seismic faults and coastal fluvial plains.
Hillshading settings are as follows:
- Light source altitudes: 60 (by default) or 45 degrees
- Light source azimuth: 45 (by default), 0 or 315 degrees
- Elevation vertical exaggeration: 0.8 (by default), 1.4 or 2.8
Step 6: Making shaded-relief basemaps (30m resolution)
Compare examples to appreciate the difference between 90m and 30m resolutions (OSM streets overlaid)
More worldwide examples with different styles
The making of mapping tilesets (e.g. Google Map Tiles, Bing Map Tiles and OpenStreetMap - OSM Tiles): the UK1, the UK2
Spatial resolution comparison
1. Data product summary
|Coverage||Global (Longitude 180W-180E,
Latitude 90N-90S), including SRTM data for
Longitude 180W-180E, Latitude 60N-56S region
|Projection||Latitude/longitude Geographic, WGS84|
|Resolution||90m / 30m
X 216,000 pixels for global raw data and
1,296,000 X 648,000 pixels for world shaded-relief basemaps
|Format||JPEG2000, GeoTIFF, ENVI's IMG or any other format major GIS software packages support|
file for global elevation data resampled at
- 173GB for the elevation raw data mosaic with gaps filled
- 5 to 50GB for each shaded-relief basemap (under JPEG2000 compression)
SRTM mosaic mask showing pixels with elevation voids (gaps) in the raw SRTM tiles
|Media||External hard drives with USB 2.0 / 3.0 compliant interfaces|
With the raw elevation data at hand, any users (even with little GIS background) can conveniently subset any small-sized region of interest and then perform dedicated 2D and 3D terrain modelling in GIS. For example, we use the low-cost GIS software Global Mapper to easily make numerous 3D renderings:
|3D: Shaded relief
Region: Mt. Fuji, Japan
|3D: SRTM elevation||3D: Global elevation mosaic + Earth Land Surface 2000 (version 1) satellite imagery. Region: Mt. Rainier, Washington|
- Some business applications (e.g., real estate, 3D urban model) require very detailed imagery such as StreetView and oblique aerial photos, but for many applications such as landscape simulations and regional environmental studies, medium-resolution data from 14.25m to 90m-resolution would be sufficient.
- Imagine you have infrastructure and platform to make another virtual world, but are seeking a low-cost solution for quality, highly-processed global imagery and terrain data.
- Imagine the enormous expertise and resources that
are needed to reproduce the similar data products.
No time is to be wasted.
- Are you thinking of boosting company-wide mapping productivity using globally consistent data sets?
- Do your mapping applications require a global coverage?
- Will the imagery and terrain data be used by someone with little or no geospatial experience?
- Imagine you are interested in GIS desktop mapping and wish to make underlying imagery and elevation layers fully dynamic and integrative.
- Imagine you do not wish to have access to or are feed up with periodic subscriptions to online imagery tiles from virtual globes such as Google Maps, Bing Maps and ArcGIS Online, due to concerns over complex licensing terms, sluggish access speed, unexpected network disconnections and lack of your own control ...
Others - Free, efficient tools for viewing large-sized JPEG2000 files, e.g.
Nowadays almost all popular GIS and remote sensing software can display JPEG2000 files easily.