I would like to automaticaly identify shadows on Landsat scene in mountaineus area using Hillshade Tool (ArcGIS 10.1). I have

  • 1 Landsat image from one date
  • DEM (SRTM) from the same area

To create Hillshade raster (ArcGIS-> Tools-> Spatial analysis -> Surface -> Hillshade) I have to insert DEM and two characteristics of actual sun position:enter image description here I have to input Azimuth and Altitude, relevant to actual sun position. In ESRI (http://resources.arcgis.com/en/help/main/10.1/index.html#//009z000000z2000000) help I can find this information:

  • The azimuth is the angular direction of the sun, measured from north in clockwise degrees from 0 to 360. An azimuth of 90º is east. The default azimuth is 315º (NW).
  • The altitude is the slope or angle of the illumination source above the horizon. The units are in degrees, from 0 (on the horizon) to 90 (overhead). The default is 45 degrees.

In different help (http://landtrustgis.org/technology/advanced/shaded%20relief) I can find information that I have to "Play with these values until you find a combination that yields reasonable, legible, and attractive results (a good starting point is altitude=45, azimuth=315)."

My question is, if it is possible to use Hillshade tool to exclude hillshades from Landsat scene? For my Landsat image I have a specific DOY (Day of Year), can´t I directly convert this DOY information in Azimuth and Altitude values? I suppose these values are relevant to the time of day but I think I can find this information somewhere on NASA web site (hour of Landsat data acquisition, not only the date).

Find shadows on Landsat image using DEM hillshade (by ArcGIS Hillshade Tool) enter image description here

Thanks for every recommendation

  • Here is an interesting, related read from PE&RS: asprs.org/a/publications/pers/2001journal/july/… – Aaron Dec 8 '14 at 15:21
  • @Aaron, reading this study, do you think that converting Landsat scene into vegetation indices (as NDVI, 5/4, ...) will be sufficient to reduce topographic effect on Landsat scene? without using any other topographic correction? thanks again, – maycca Dec 9 '14 at 14:20

I would discourage you from perusing the course that you are on. In most cases there is some degree of useful reflectance information in every pixel. Before applying a dubious technique that just excludes what is perceived as shadow, you could first attempt to correct for the effect.

I have had consistent success with the Minnaert correction in very steep terrain and have implemented it in multiple raster algebra environments. There are several other robust options available as well. You will most certainly require the header information for the scene. You can obtain a header from Earth explorer.

Here are a few papers to get you started.

Pixel-based Minnaert Correction Method for Reducing Topographic Effects on a Landsat 7+ETM Image

Comparison of Topographic Correction Methods

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