My PhD thesis focuses on spatio-temporal river change by aerial photos and satellite images. I need the accurate width of river at 60 years ago until know. The aerial photos scale that I have are 1:50000 for 60 years ago and 1:20000 for 30 years ago and IRS and ETM data for present.
if you want to map rivers accuratley i suggest to to have at least 3 pixels for the river width. in this case -> 15 m, the data should feature approximatley 5 m ground sample distance (GSD)
Hi Saleh,
Some time ago as a part of my PhD I used an optical micrometer to measure river channel widths from aerial photos - an easy, rapid and successful technique (see abstract and reference to journal article below). If your river is 15 m wide as the title to your question suggests, it would be best if you could obtain the highest resolution satellite imagery possible (e.g. quickbird, ikonos). Below is the abstract and reference to the paper that does contain information relevant to your study. If you are interested in a copy, let me know.
Cheers
Ivars Reinfelds
Reconstruction of changes in channel width
Measurements of bankfull width determined from aerial photography are compared to measurements determined from surveyed river cross-sections for a low-gradient, sinuous, lowland alluvial river. Excellent agreement between independent datasets suggests that along such rivers downstream and/or temporal changes in bankfull width can be quickly and accurately reconstructed from aerial photography without the need for a large number of surveyed channel cross-sections. Guidelines are provided as to the minimum width of river channels that can be accurately measured with an optical micrometer from aerial photography, and the number of measurements necessary to determine accurate ‘average’ values for comparisons of bankfull width between reaches along the same river channel, and for comparisons between separate rivers.
Applied Geography - APPL GEOGR. 01/1997; 17(3):203-213.
Dear Saleh, the answer is highly depends on your accuracy needs. If your river is cca 15 meters wide as suggested by the question, 1-2 meters of pixel size is needed. This means ultra-high resolution satellite data (ikonos, quickbird), which are quite expensive. Aerial photographs, scanned or digital if available, can be a perfect solution. The problem with them is mostly the ortho-rectification, especially in areas without good control points. The start of processing, however, is the knowledge of camera data (focal length, location of the fiducial points in the film plane) to provide a camera model to your GIS processing software.
Also, I have to agree with Ivars: the season of the aerial photograph making is important as well as the climate of the area of your target river.
Dear Gobar
Thank you so much for your interesting and useful explanation.
Best Regards
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