How To Make a Map of a River at EVS Precision
African River
How do you make a map of a river? The same way you create a map of an island, carefully and with precision. I have made river maps before and they are challenging. Riverine shorelines are more dynamic then island shorelines. They have these swampy shores that are half river and half shore. Where does the river end and the shoreline begin? Rapids offer a different challenge. Are you mapping rapids formed by below water features or above water features? How many of the above water features should you digitize? And riverine islands are most often half river and half shore continuing the challenge of river mapping.
My real objective was to determine a reasonable length of river that I might be able to digitize in a typical work day. It turns out that this 150-kilometer stretch of river took one work day to complete.
Now for the "How To" part. First you must determine the resolution you will work from to create the maps at the scale your client requires. Working from Landsat ETM+, I determined that 1:12,500 was a good resolution to work from to create 1:25,000 scale river shorelines. For some features I work at 1:6,250. The digitizing is a slow steady progression of vectors being placed on the shoreline image. The order of feature digitizing is one shoreline and then the other moving in down-river direction. Finally, the riverine islands and above water features are digitized. The secret is to work steady and not try to do too much with the shoreline.
Digitizing the River Shoreline
If the digitizing line (yellow line), which is about 5-meters thick (on-screen), covers the indentation make a straight line. If the shoreline indentation extends beyond the digitizing line, follow the indentation.
Do you realize how poorly mapped most rivers are? The only consistent river mapping that I am aware of at 1:25,000 scale is country topographics. Few of these are digitized and the costs are prohibitive. The Digital Chart of the World (DCW) rivers consist of thin blue lines, not good for detailed studies. Navteq shows rivers that are roughly approximations of the underlying rivers as depicted on Landsat. However, they aren't EVS precision. However, rivers, such as the one in this post mapped at EVS precision are as good as the base imagery they are taken from.
Looks like a great undertaking!
Just out of curiosity, what is the methodology you are using to correlate imagery resolution and zoom level to digitized accuracy? It looks like you stay at a consistent zoom level and pan from scene to scene, which would allow a computed accuracy and precision value.
With regard to inaccurate streams, there may be many reasons for this - shorelines tend to vary quite a bit from season to season, and from year to year, particularly if there have been flood events or in flat areas, such as river deltas. Or are you more typically finding shorelines which are wholly misplaced (such as outside of the observed floodplain?
Posted by
Dave Smith |
9/04/2006 12:01:00 PM