[Xastir] Re: Tons of Tiger Maps -- can we use them?

[Gerry Creager N5JXS]

Hmmm.  Sounds like your area hasn't been reflown recently with digital 
acquisition.  USGS is updating large areas of the country, but they 
refly rapidly growing urban areas more frequently than rural/ag areas or 
slow-growing urban regions.

Another potential problem: In the old days, hand digitization off large 
format aerial photos was the norm and a lot of the work was done by 
interns and summer-hires, often high school kids.  Let's say that 
digitization accuracy was often better in August than June, and there 
was no accounting for some digitizing after big parties...  Finally, 
recall that the aerial photos were only as good as the days they were 
flown on.  When the kids digitized them, they often had to guess about 
what was underneath that cloud or canopy cover.

73, gerry

Tom Russo wrote:
> On Tue, Nov 25, 2003 at 10:36:33AM -0600, a Mr. Richard Feyler of Fort Lee, New Jersey <gerry.creager at tamu.edu> writes 'Dear Rosanne Rosannadanna':
> 
>>Warning!  Warning Will Robinson!  Warning!  Datum error!
>>
>>300m in NM sounds like a NAD27/NAd83 offset.
> 
> 
> Well, it almost does if taken out of context.  300m in my area exceeds the
> normal datum shift which is at most 200m.  Plus I didn't actually say 
> "all the streets are shifted" but rather "my street is shifted."  Fact is, 
> the tiger map is just not accurate in my area.  Some streets line up perfectly, 
> others bear no resemblance whatsoever to the streets whose names they bear --- 
> mine is one, as it's not only shifted, but completely the wrong shape.  
> 
> Overlaying the tiger map on any other raster or line map of the area 
> shows *some* agreement, but not consistent error or shifting.  The USGS and
> USFS DLGs and CFFs of the same area line right up on top of the terraserver
> photos of my area, but whether a tiger line will line up with a feature 
> seems to be hit or miss.  Larger roads are mostly right, small dirt roads
> are very, very wrong or completely accurate.  Their field-checker might have
> been sampling the local mushrooms.
> 
> T.

-- 
Gerry Creager -- gerry.creager at tamu.edu
Network Engineering -- AATLT, Texas A&M University	
Cell: 979.229.5301 Office: 979.458.4020 FAX: 979.847.8578
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