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Re: st: spatial analysis - DISTANCE


From   David Torres <torresd@umich.edu>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: spatial analysis - DISTANCE
Date   Sun, 30 Aug 2009 12:23:37 -0400

This is what I get:

. clear

. set obs 9
obs was 0, now 9

. gen id =_n

. set seed 1234

. gen latitude = uniform()/2

. gen longitude = uniform()/2

.
. mata
------------------------------------------------- mata (type end to exit) --------------------------------------------------
: X= st_data(.,( "latitude" , "longitude"))

: X =X*(pi()/180)

: dist = J(rows(X),rows(X), 0)

: for (i = 1; i <=rows(X); i++) {
    for (j = 1; j <=rows(X); j++) {
          dist[i,j] = (1/1.609)*6372.795*(2*asin(sqrt( sin((X[i,1] ///
          - X[j,1])/2)^2 +  cos(X[i,1])*cos(X[j,1])*sin((X[i,2] ///
          - X[j,2])/2)^2  )))
          }
}

: N = mm_cond(dist :<= J(rows(dist), cols(dist), 10), ///
J(rows(dist), cols(dist),1), J(rows(dist), cols(dist), 0))
                 <istmt>:  3499  mm_cond() not found
r(3499);

: st_matrix("N", N)
                 <istmt>:  3499  N not found
r(3499);

: dist
[symmetric]
1 2 3 4 5 6 7 8 +-----------------------------------------------------------------------------------------------------------------
  1 |            0
  2 |  5.157033248             0
  3 |  21.21062478   21.71534226             0
  4 |  26.95060264   27.64838102   5.939930861             0
  5 |   16.1570544    21.1565295   30.29833499    34.5960745             0
6 | 14.73674249 19.76346139 22.78331973 26.58113711 8.439103366 0 7 | 6.614080612 9.633040063 15.1934957 20.70923619 17.14313644 12.30209368 0 8 | 22.069818 20.34155009 9.309128607 12.92307095 35.0470512 28.79024435 18.03555849 0 9 | 20.67616548 20.42184227 3.226163273 8.078752624 31.37740006 24.32651125 15.24958295 6.116268222 +-----------------------------------------------------------------------------------------------------------------
                 9
     ---------------+
  9              0  |
     ---------------+

: end
----------------------------------------------------------------------------------------------------------------------------

. svmat N
matrix N not found
r(111);

end of do-file

r(111);

.

There are two 3499 errors and two 111s.

So I need to store my unitids (actually school ipeds codes) so that I can eventually collapse that data on census 2000 info to investigate what postsecondary schools and types of schools serve what populations. If I take distances from school to all tracts within, say, 10 miles, I want to collapse population demographics and create some descriptives using stata. I can do this the other way around, too, using tracts as the center of my radii. I can get a return on the one nearest school, but can't seem to get the next nearest on out to the edge of the specified radius.

Diego

--------------------------------------------

David Diego Torres




Quoting Scott Merryman <scott.merryman@gmail.com>:

Sorry - I forgot to mention the code uses Ben Jann's -moremata-.

What is the error?

The example returns the distance matrix and neighbor matrix and then
creates the N variables in the original data set:

: dist
[symmetric]
                 1             2             3             4
  5             6             7             8             9
+--------------------------------------------------------------------------------------------------------------------------------+
  1 |            0
                                                             |
  2 |  5.157033248             0
                                                             |
  3 |  21.21062478   21.71534226             0
                                                             |
  4 |  26.95060264   27.64838102   5.939930861             0
                                                             |
  5 |   16.1570544    21.1565295   30.29833499    34.5960745
  0                                                          |
  6 |  14.73674249   19.76346139   22.78331973   26.58113711
8.439103366             0                                            |
  7 |  6.614080612   9.633040063    15.1934957   20.70923619
17.14313644   12.30209368             0                              |
  8 |    22.069818   20.34155009   9.309128607   12.92307095
35.0470512   28.79024435   18.03555849             0                |
  9 |  20.67616548   20.42184227   3.226163273   8.078752624
31.37740006   24.32651125   15.24958295   6.116268222             0  |
+--------------------------------------------------------------------------------------------------------------------------------+



+-----------------------------------------------------------------------+ | id latitude longit~e N1 N2 N3 N4 N5 N6 N7 N8 N9 | |-----------------------------------------------------------------------| 1. | 1 .23851 .0647102 1 1 0 0 0 0 1 0 0 | 2. | 2 .1653217 .0502572 1 1 0 0 0 0 1 0 0 | 3. | 3 .1722953 .3643155 0 0 1 1 0 0 0 1 1 | 4. | 4 .1788673 .4499913 0 0 1 1 0 0 0 0 1 | 5. | 5 .4713051 .043847 0 0 0 0 1 1 0 0 0 | |-----------------------------------------------------------------------| 6. | 6 .4298635 .1586816 0 0 0 0 1 1 0 0 0 | 7. | 7 .251903 .1594484 1 1 0 0 0 0 1 0 0 | 8. | 8 .0454795 .3190092 0 0 1 0 0 0 0 1 1 | 9. | 9 .1304665 .3436173 0 0 1 1 0 0 0 1 1 | +-----------------------------------------------------------------------+



Why do you want to store the neighboring unit ids?

Scott

On Sun, Aug 30, 2009 at 10:40 AM, David Torres<torresd@umich.edu> wrote:
I copied and pasted your example, Scott, and got a couple of error messages
about N matrix not found.  I installed the moremata files, but this didn't
solve the problem.  I'm not quite as adept at using stata as some, so,
please, bear with me.  I've never used a mata command, so would like to know
whether its use will allow for variable creation wheere the unitids can be
stored and reshaped and all that jazz?

Thanks,
--------------------------------------------

David Diego Torres, MA(Sociology)

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