- freely available
ISPRS Int. J. Geo-Inf. 2016, 5(5), 64; doi:10.3390/ijgi5050064
Abstract: Geo-tagged photographs are used increasingly as a source of Volunteered Geographic Information (VGI), which could potentially be used for land use and land cover applications. The purpose of this paper is to analyze the feasibility of using this source of spatial information for three use cases related to land cover: Calibration, validation and verification. We first provide an inventory of the metadata that are collected with geo-tagged photographs and then consider what elements would be essential, desirable, or unnecessary for the aforementioned use cases. Geo-tagged photographs were then extracted from Flickr, Panoramio and Geograph for an area of London, UK, and classified based on their usefulness for land cover mapping including an analysis of the accompanying metadata. Finally, we discuss protocols for geo-tagged photographs for use of VGI in relation to land cover applications.
2. Protocols for Existing Geo-Tagged Photograph Sites and Inventory of Metadata
2.1. Current Protocols for Geo-Tagged Photographs
2.2. Inventory of Metadata for Geo-Tagged Photographs
3.1. Study Area
3.2. Use Cases
3.2.1. Metadata Requirements of the Use Cases
3.2.2. Analysis of Metadata
3.2.3. Analysis of Content Usability
4.1. Metadata Requirements of the Use Cases
4.2. Analysis of Metadata
4.3. Usability of Photographs Based on Content Analysis
Conflicts of Interest
Application Programming Interface
CORINE land cover
Degree Confluence Project
International Press Telecommunications Council
Land use land cover
Minimum Mapping Unit
Open Geospatial Consortium
Volunteered Geographic Information
World Wide Web Consortium
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|Social networking/sharing of all kinds of information||Facebook/Google+||Minimum: None |
Optional: Tag friends; add comments; add location automatically from the photographs, if present.
|Foursquare||Minimum: None other than the photograph must be linked to one of four objects, e.g., the venue or a tip provided by the venue. |
Optional: Coordinates (and accuracy), altitude (and vertical accuracy), up to 200 characters of text to accompany the photograph.
|Minimum: Need to upload photographs to a “board” and add a description to the photograph. |
Optional: Add location on a map; tag friends.
|Minimum: Text from the tweet, which might describe the photograph. |
Optional: Tag users, enhance and filter photographs, location.
|Photograph sharing sites||Flickr||Minimum: None |
Optional: Title, description, tags, location. Assign photograph to a Flickr Group where there is internal moderation of the photographs’ theme.
|Minimum: None but can only upload from mobile devices; EXIF data are removed from the photographs before upload unless saved to a person’s Photo Map. |
Optional: Effects or filters can be added; a caption; location information.
|Panoramio||Minimum: None. |
Optional: Title, comment, tags and location.
|Picasa||Minimum: None. |
Optional: Captions, manually locate photos using Google Earth.
|Documenting Landscapes||Degree Confluence Project||Minimum: X, Y location, resolution 600 × 400 pixels 16-bit; single view shots; 2 pictures from the confluence (within 100 meters of the confluence); brief description of the confluence and the surrounding area. |
Optional: Photograph documenting the GPS acquisition (WGS84 position, altitude, reported error, and date/time); 4 pictures taken in the cardinal directions (N, S, E, W), or one or more panoramic views from the confluence; 1 picture of the general area of the confluence.
|Geograph||Minimum: 480 pixel long edge as jpg only but optimal is 640 pixels long edge; grid reference of 1km square; position of the photographer; position of the subject; title for the photograph; geographical context (must click one category); accept the terms of conditions of the CC license. The date taken is read from the photograph along with the data uploaded, and the view direction is calculated. |
Optional: A more detailed description/comment can be added after the title; as many of the geographical context tags can be selected; optional tags can be added.
|Oklahoma Field Photo library||Minimum: X, Y location. |
Optional: LULC category from a dropdown list; orientation (from 8 categories); a description field of more detailed LULC or other ancillary information about the photograph.
|Pictures Geo-Wiki||Minimum: X, Y location, direction/orientation, tilt, offset in meters (adjusted manually in user settings), date taken, accuracy of GPS, some limited information about the photographer from the initial registration on Geo-Wiki, land cover tags (or per-set tags from user constructed legends) |
Optional: Additional comments/tags can be added.
|Professional in-situ data collection||Land Use/Cover Area frame Survey (LUCAS) (Eurostat)||The protocol is very detailed but this description deals only with the photographs. |
Minimum: A photograph of the LUCAS point is taken and should contain a stable landmark. A marker is placed on the point if the point is reachable. Four photographs are then taken in the mandatory order of N, E, S and W. Land cover and land cover percentage is specified from a predefined legend.
Additional: Additional photographs are taken of irrigations systems, transects, soil, reasons of why the photograph could not be reached, or photos that complement the required ones (if relevant).
|Location information||Photograph with location information, a place name or x,y coordinates||√||√||√|
|Direction/Orientation||Compass direction and precision||X||X||√|
|Tilt||Applicable if taken with a smartphone/tablet||X||X||X|
|Offset (in meters) of the subject of the photograph||How far in meters is the object being photographed away from the photographer||X||X||X|
|Date uploaded||The date the photograph was uploaded to the application||√||√||√|
|Date taken||The date the photograph was taken||√||X||√|
|Weather||Information about the weather conditions when the photograph was taken||X||X||X|
|Method of georeferencing||Manually located on a map, automatically matched using image processing or device-enabled positioning (e.g., GPS-enabled device, wifi/IP positioning)||X||X||X|
|Type of GPS-enabled device||Make and model of: GPS, smartphone, tablet, camera with built-in GPS||X||X||X|
|Accuracy of GPS-enabled device||Accuracy in meters||X||X||X|
|Focal length||In mm, which provides an indication of the zoom level||√ *||√ *||X|
|Reference length or area||Presence of a reference length or area on the photograph, e.g., a measuring stick||X||X||X|
|Type of tags present||None, fixed categories, freeform or mixed||Freeform||Freeform||Mixed|
|Tags||The tags that accompany the photograph||√||√||√|
|Description||Text describing the content portrayed in the photograph.||√||√||√|
|Title||Title of photograph||√||√||√|
|Different directions||Requirement to take the photograph in either four cardinal directions or panoramic||X||X||May be present|
|Information about the photographer||Age, gender, expertise, relationship to other photographers, home location of photographer (country, place, XY coordinate), if part of an online photo sharing website/system then perhaps the number of photographs the photographer has, the number of groups he/she is involved in, status and kudos indicators||Some may be present||Some may be present||Some may be present|
|Licensing||Openly available with no license, type of license, e.g., Creative Commons and the level of use, for private use only||√||√||√|
|Rule Number||Rule Description|
|1||Land cover is only considered when it is within about 10 m of the photographer, to take into account positioning errors of the photograph. Thus, land cover types in the far distance should not be considered.|
|2||If it is possible to see or infer with reasonable certainty what is at the photographer’s footprint (even when the footprint is not visible), and there is only one possible class from the list indicated in Section 3.2.3, choose “Yes”.|
|3||If more than one of the classes above can be assigned to the photographer’s footprint vicinity (using the 10 m limit defined in 1), choose “maybe”.|
|4||If there is no information about what may be at the photographer’s footprint, e.g., an aerial or panoramic view, then choose “no”.|
|5||Individual trees are discounted regarding the dominant land cover (e.g., a tree in a grass field) unless one can infer from the photograph that there are many trees around.|
|6||For vintage photographs, the answer is “no”, since the land cover may have changed (or the photograph may be incorrectly geo-tagged).|
|7||For snow that completely covers the surface (so it is unclear what the underlying land cover is), because the study area is in London, the answer should be “no”. Here context is used, not only the photograph, because in the city of London it is known that no permanent snow cover exists.|
|8||For photographs taken underground, i.e., in a metro station, the answer is “no”. If the station is clearly above ground and there is no other land cover type within 10 m, then the answer is “Yes” (artificial surfaces).|
|9||Water frequently causes difficulties because in many cases it is not possible to unequivocally determine if the photograph was taken from a boat (then the answer should be “Yes”), on a bridge, or at the water vicinity. Then, if the water is identified to be within 10 m of the photographer, the answer is “maybe”.|
|Metadata Requirements||Use Case 1 (Classifiers Training)||Use Case 2 (LU/LC Map Validation)||Use Case 3 (Complement Validation)|
|Essential||Location information, Date (uploaded or taken)||Location information, Date taken, Method of georefencing (i.e., by GPS-enabled device)||Location information, Date (uploaded or taken)|
|Desirable||Tilt, Direction/Orientation, Offset (in meters) of the subject of the photograph, Method of georeferencing (any method for Use Cases 1 and 3), Type of GPS-enabled device, Accuracy of GPS-enabled device, Focal length, Reference length or area, Type of tags present, Tags, Title, Description, Different directions, Licensing, Information about the photographer (for Use Case 2)|
|Unnecessary||Weather, Information about the photographer (for Use Cases 1 & 3)|
|Tags||Descriptions Geograph||Titles Flickr|
|Number of tags/words in descriptions/words in titles||Mean||4.5||9.8||2.8||18.9||3.9|
|Number of photographs||344||696||787||628||927|
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