Digital photographs were something obtained only by orbital satellites, such as Landsat and Spot. Today, photos taken with compact or SLR digital cameras are found in an ever increasing number of applications. In a survey made in the USA and Europe in 2008, 27% of the respondents informed to have between 1,000 and 5,000 digital photographs, while 11% claimed to have more than 10,000 digital photos.
The geo-referencing of digital photographs was also done only for geospatial imagery, such as those acquired by orbital satellites or airborne systems. However, there is an increasing demand for geo-referenced photos taken with digital cameras. In 2008, the popular service to share digital photographs Flickr informed that more than 2.5 million of georeferenced photos were up loaded to the site every month.
The geo-referencing of photographs taken with digital cameras was a labor intensive process until the introduction by Ricoh of the first camera with GPS in 2003. Until then, this was a manual or semi-automated process.
A digital camera plus GPS equipment were used in these processes. The geo-referencing was done manually or through software capable of synchronizing the data from both equipment using the time registered by them.
Geo-referenced digital photographs will be more present in our lives due to the great demand for information and services based on geographical location. Makers of digital cameras are more often integrating GPS receptors in their cameras to automate the process of geo-referencing digital photographs utilizing only one piece of equipment. Around 2007, we also see the release of small devices with GPS receptors, known as geotaggers, which can be carried by photographers to register the GPS positioning of their digital photographs later by a semiautomated process, as above-mentioned. Additionally, there already has been released GPS modules made by third parties for a few of the Canon, Fuji and Nikon cameras in 2009.
Camera’s Precision
As different GPS devices utilized for mapping and navigation have different levels of positioning accuracy (metric, sub-metric, etc.), the GPS modules integrated with digital cameras can also present different quality of the GPS positioning data. This article reports the results of a study conducted to determine the accuracy of the GPS positioning of two digital cameras with integrated GPS. The two digital cameras with GPS used in this study were the Ricoh Caplio 500SE and Nikon Coolpix P6000.
Accuracy is the conformity of an experimental measure to a standard or a true value. In this case, the experimental measure is considered to be the positioning data obtained by the GPS of the cameras at the geodetic stations, and the standard, or true, value would be the precise GPS positioning data reported by IBGE for each one of these stations. The conformity of the two values is the calculated the geodetic distance between the two pieces of GPS positioning data.
Therefore, the methodology used in this study to make an evaluation of the accuracy of the cameras’ GPS positioning was to determine the geodetic distance by the method of Vincenty between the positioning coordinates (latitude and longitude) obtained by the cameras’ GPS at 29 geodetic stations of IBGE (Planimetric Stations – SAT) and the precise positioning coordinates of these stations.
The positioning coordinates obtained by the cameras’ GPS are stored in the header of the image file in JPEG format as Exif 2.2 tags. The Ricoh 500SE camera has also the capacity to store altitude and azimuth (orientation) data as Exif 2.2 tags, as well as write this data on the image at the time that the photograph is taken. Besides this data, the Ricoh 500SE can also store 10 attribute data per photograph.
The cameras’ coordinates are stored in WGS-84 while the GPS positioning coordinates of the IBGE’s geodetic stations are reported in SAD69 and Sirgas2000 in both projections latitude/longitude and Universal Transverse Mercartor (UTM). Although there is only a small difference between WGS-84 and Sirgas2000, both coordinates were transformed to WGS-84 lat/lon utilizing the software Geographic Calculator 7.3 from Blue Marble Geographics prior to the distance calculation.
The precise GPS positioning coordinates of the geodetic stations are described by IBGE in a report for each station. The GPS positioning data obtained by the cameras Ricoh 500SE and Nikon Coolpix P6000 were determined by placing the cameras at each Planimetric Station – SAT. The cameras remained placed at the stations for a few minutes in order to stabilize the tracking of GPS satellites and determine the positioning coordinates.
The visited planimetric stations were materialized on the terrain through the use of pyramidal and cylindrical pillar bench marks made out of concrete according to IBGE standards. A tripod was used to center the cameras on the pyramidal bench marks.
The cameras were positioned on the center of the centering device of the cylindrical concrete pillar bench marks.
After visiting all planimetric stations, the coordinates obtained by the cameras’ GPS were extracted and calculated the geodetic distance of these coordinates with those from the geodetic stations. The average accuracy was also computed for each camera. The operations to extract the cameras’ coordinates and the distance calculation, according to Vincenty method, were done through the use of software developed in C by GeoDesign Internacional for this study.
According to IBGE, the Brazilian Geodetic System (BGS) is composed of the altimetric, planimetric and gravimetric networks. The BGS is made out of the stations materialized on the terrain, which positions serve as precise reference for many engineering projects (construction of roads, bridges, dams, etc.), mapping, geophysics, and scientific research, among other applications.
The Planimetric Stations – SAT selected for this study are located in the States of Minas Gerais, Rio de Janeiro and Sao Paulo, within the coast, Paraiba do Sul River Valley and Mantiqueira Mountain regions. The altitudinal variation in the study region ranges from 4.34 to 1,237.36 meters. The stations were situated in planes, mountainous areas and small valleys within the region of study.
Results
The results show a variation in the accuracy of the GPS positioning between 0.38 to 4.84 meters for the Ricoh 500SE, and from 1.26 to 432.41 meters for the Nikon Coolpix P6000. The computed average accuracy is 2.53 meters for the Ricoh 500SE and 24.80 meters for the Nikon Coolpix P6000. The Ricoh 500SE also presents an average difference of 5.38 meters in the altitude measurement. The Nikon Coolpix P6000 does not record altitude data.
The Nikon Coolpix P6000 failed to get a GPS fix in four of the photographs taken at the 29 Planimetric Stations – SAT of IBGE, or about 13.8% failure. It should be observed that during this study there were 117 photographs taken with the Nikon Coolpix P6000 when it failed to get a GPS fix in 22 situations, which corresponds to 18.8% failure.
The results of the accuracy for both cameras are within the expected values. The Ricoh 500SE camera has a GPS module with superior capacities than the one of the Nikon Coolpix P6000 due to its better accuracy, low acquisition time of GPS satellites signal, reliability of position tracking in real time, and log of the camera’s altitude and orientation (azimuth).
The failures in the reception of the GPS signal by the Nikon Coolpix P6000 were due to environmental conditions while taking the photographs, such as overcast skies, topographic obstruction and presence of vegetation cover.
Both cameras offer excellent photography features and their images are of the highest quality. The Nikon Coolpix P6000 presents a higher resolution than the Ricoh 500SE, that is 13.5 versus 8.3 Mega pixels, respectively. On the other hand, the body of the Ricoh 500SE is rugged and designed for fieldwork, being rubberized, water resistant, dust proof and could be dropped from 1 m in concrete surface.
The Ricoh 500SE can be used in fieldwork under adverse environmental conditions to integrate digital photographs in large scales mapping works. Due to its feature to write the GPS positioning, orientation and attribute data on the image at the time of taking a photograph, the Ricoh 500SE can also be utilized to register photographs for works of examination by experts, since these data are written by the closed system of the camera at the time of taking the photograph instead of later through the use of asoftware.
The digital photographs taken by the Nikon CoolpixP6000 should be employed in low scales mapping andgeneral use, where there is no need for quality GPSpositioning data, such as tourism, adventure and realstate, among others.
Reinaldo Escada Chohfi
Director, GeoDesign Internacional
www.geodesign.com.br
reinaldo@geodesign.com.br