Making Shaded Area Polygon from HILLSHADE raster in ArcGIS Desktop?

Question

How am I suppose to extract the values ( Value<=100 ) from this HILLSHADE raster, and then make the extracted values as a VECTOR POLYGON (shapefile) ---I'm using ArcGIS 9.3 (ArcInfo with all extensions)--- Can you please give me the work flow.

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Answer 1

1. Use Reclassify to turn "0 - 100" into "1" (or any other value, really), and "100.0001 - 999" into "NoData". (I'm assuming you've got Spatial Analyst, since you made a Hillshade raster.)
2. Convert Raster to Polygon.

I like this for simplicity, but you lose the values in the raster. If it's important to retain the values, (e.g., the final polygon knows that one area was 75 and another was 80):

1. Use Raster Calculator with the expression: Con("temp0123" <= 100,"temp0123") (anything <= 100 is retained, and keeps its value)
2. Convert Raster to Polygon.

Answer 2

I would suggest first reclassifying your hillshade into values 0-99 and 100-255 using 3D Analyst Tools - Raster Reclass - Reclassify. You can then convert your output to a polygon using Conversion Tools - Raster to Polygon and selecting 'Value' as the Field (this should be the default). Depending on the complexity of your hillshade, and the distribution of your target values, you may then want to dissolve the new polygon based on the value field using Data Management - Generalization - Dissolve. This will give you two, multipart polygons representing the two value ranges you initially used in your reclassification. From this you can select, display, or export the polygon representing values >100.