Creating a single geospatial source of truth for South Africa

March 15th, 2018, Published in Articles: PositionIT

Geospace, a digital aerial imagery service provider and Hexagon Geospatial Premium Partner, uses Hexagon’s powerful GeoCompressor software to create and maintain a single seamless (ECW) digital mosaic of the country utilising the imagery from the Chief Directorate: National Geo-spatial Information’s (CD: NGI) annual aerial survey programme.

The company’s customers – including the CD: NGI, the South African National Roads Agency, and Statistics South Africa – provide mapping census, transportation, and employment data for the populace of South Africa. Naturally, this data requires an accurate, current imagery basemap of the region. Not only does this provide the best backdrop, but the context provided by this imagery can be vital in understanding and drawing conclusions from the data.

One basemap, multiple uses

When covering a country of 1,22-million km2 at 50 cm GSD resolution, the amount of data generated can be staggering. Each of the approximately 440 flight missions captures 500 individual images and covers areas of about 2700 to 3000 km2. After orthorectifying and mosaicing the images to a scale of 1:10 000, Geospace is left managing nearly 46 000 files.

This amount of data causes storage, management and display problems. Each 1:10 000 mosaic in uncompressed tiff format is approximately 500 MB in size. Combined, the 46 000 files are 30 TB in size, which Geospace and its clients found very difficult to work with.

Fig. 1: File compression comparison, indicating a 97% savings over uncompressed imagery and 71% savings over JPEG-compressed imagery.

Fig. 1: File compression comparison, indicating a 97% savings over uncompressed imagery and 71% savings over JPEG-compressed imagery.

Using Hexagon Geospatial’s GeoCompressor, Geospace was able to solve the file-size and disk-space problems. The software compressed all 46 000 files at 50 cm GSD into a single, seamless mosaic in ten days, shorting the usual two-week wait. The resulting ECW image is compressed by a factor of ten, maintaining visually-lossless quality.

Working with a single seamless mosaic also saves time, disk space and money, and the company’s clients are very satisfied with the performance and manageability of the final ECW image.

Since the CD: NGI acquires fresh imagery for new areas every month, the dataset needs to be updated constantly. This is something the GeoCompressor solution also addresses. In addition to compressing the files into a single mosaic, the software provides simple tools to update a particular region within an existing ECW file without requiring Geospace to recreate the entire mosaic. This streamlines the workflow and ensures that the latest data can be rapidly included in the mosaic without a large processing burden.

Simplified distribution

With so many different agencies making use of the basemap, the final file has to be quickly disseminated to thousands of users simultaneously. At CD: NGI, Sanral, and Statistics South Africa, the ECW image is streamed to end users on desktop GIS and web GIS technologies via Erdas Apollo, a Hexagon Geospatial server product. The server product offers interoperable OGC services like WMS and WMTS, which guarantee that every standard GIS can benefit from the centralised geoservice.

In 2017, the imagery programme began requiring that all new photography be taken at 25 cm GSD resolution, which will result in a total image size of over 100 TB uncompressed.

Geospace covered 240 000 km2 during the 2017/18 financial year. The entire country will be covered by 2020 through the CD: NGI survey programme. The company will start processing the seamless 25 cm mosaics for each province until the entire country is covered, after which it plans to create another single ECW file for the resulting image.

This single ECW file therefore provides a basemap of South Africa that is a single source of truth, ready for GIS users to incorporate the latest data and for government decision-makers to analyse for a greater understanding of their challenges ahead.

In this way, using GeoCompressor and ECW as output format has significantly enhanced the company’s productivity and the cost saving on time and disk space.

Contact Hennie Loots, Geospace International, Tel 012 348-4586, henniel@geospace.co.za