Mapping our land more clearly and colourfully
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Acknowledging SRTM






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World basemaps

global basemaps



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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 2014 release.

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

Since some parts of the world (e.g. low-lying coastal regions) are below the mean sea level from physical measurement, it is often ineffective to show those land areas when ocean waters are represented by the mean sea level of zero metres. We have developed and applied a high-resolution mask to indicate land and ocean boundary, and this step is especially useful for making realistic shaded-relief basemaps.

Step 5:
Making shaded-relief basemaps (90m resolution)

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.

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)

We are able to interpolate the global digital elevation model from the resampled 90m resolution to 30m resolution (up to the terabyte size in raw binary format), followed by the making of a series of unique shaded-relief basemaps for the entire globe. A total of three styles are developed. Each basemap is with the image size of 1,296,000 X 648,000 pixels.

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

resolution comparison


Left 1km-resolution in previous global elevation mosaics, e.g., GTOPO30, SRTM30
Right 90m-resolution in the new near-global SRTM elevation mosaic and hillshading maps
  1km to 90m resolution: 100 times more detailed!
1km to 30m resolution: 1,000 times more detailed!

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
Image size 432,000 X 216,000 pixels for global raw data and shaded-relief basemaps
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 size Single file for global elevation data resampled at 90m-resolution
- 173GB for the elevation raw data mosaic with gaps filled 
- 5 to 50GB for each shaded-relief basemap (under JPEG2000 compression)
Additional file

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

Are you interested in large-sized, GIS-ready, global data products and having a copy locally? Please consider the following:

  • 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.


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