Category: Uncategorized

Converting OS Grid coordinates into Lat/Long using PostGIS

I had a case of needing to convert Ordnance Survey grid coordinates like 548420,247240 that were in a table from NAPTAN into latitude & longitude so that I could display them in a map.

Most references online appear to do this by running the data through some application then importing it back into the database but as I’m using PostGIS I wanted to do it directly on the database.

So here’s how I did it.

First here’s the table I want to process:

gis=> select crscode,stationname,easting,northing from naptan limit 5;
 crscode |           stationname           | easting | northing
---------+---------------------------------+---------+----------
 ABA     | Aberdare Rail Station           |  300400 |   202800
 AUR     | Aberdour Rail Station           |  319100 |   685400
 AVY     | Aberdovey Rail Station          |  260600 |   296000
 ABE     | Aber Rail Station               |  314870 |   186950
 AGL     | Abergele & Pensarn Rail Station |  294612 |   378681
(5 rows)

Now you see we have the rail station’s coordinates as an easting & northing pair.

First I added a geometry column to the table naptan:

gis=> select AddGeometryColumn( 'public', 'naptan', 'the_geom', 27700, 'POINT', 2);
                  addgeometrycolumn
------------------------------------------------------
 public.naptan.the_geom SRID:27700 TYPE:POINT DIMS:2
(1 row)

Here 27700 is the ESRI code for the OS grid system.

Next I updated the table to create geometries for each entry:

gis=> update naptan set the_geom=GeomFromText('POINT('||easting||' '||northing||')',27700);
UPDATE 2603

This updates each row using the easting and northing columns.

Finally I needed to add two new columns for latitude & longitude and populate them with the final values:

gis=> alter table naptan add column lat real;
ALTER TABLE
gis=> alter table naptan add column long real;
ALTER TABLE
gis=> update naptan set long=st_x(st_transform(the_geom,4326)), lat=st_y(st_transform(the_geom,4326));
UPDATE 2603

That’s it, we now have the table populated with lat/long coordinates:

gis=> select crscode,stationname,lat,long from naptan limit 5;
 crscode |           stationname           |   lat   |   long
---------+---------------------------------+---------+----------
 ABA     | Aberdare Rail Station           | 51.7151 | -3.44308
 AUR     | Aberdour Rail Station           | 56.0546 | -3.30056
 AVY     | Aberdovey Rail Station          |  52.544 | -4.05707
 ABE     | Aber Rail Station               |  51.575 | -3.22983
 AGL     | Abergele & Pensarn Rail Station | 53.2946 | -3.58262

Hopefully this is correct – if it isn’t please let me know but the final data looks ok to me.

/home/edivad

If in emacs you need a different file coding system (line terminator), for example you are on a windows system and need to type a unix like text file (or vice versa), you can easily convert the buffer coding system.

Dos to unix

M-x set-buffer-file-coding-system RET undecided-unix
save the file (C-x C-s)

or

C-x RET f undecided-unix
C-x C-f

Unix to dos

M-x set-buffer-file-coding-system RET undecided-dos
save the file (C-x C-s)

or

C-x RET f undecided-dos
C-x C-f

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Generating private keys with openssl

Keys are the basis of public key algorithms and PKI. Keys usually come in pairs, with one half being the public key and the other half being the private key. With OpenSSL, the private key contains the public key information as well, so a public key doesn’t need to be generated separately.

Public keys come in several flavors, using different cryptographic algorithms. The most popular ones associated with certificates are RSA and DSA, and this  article will show how to generate each of them.

Generating an RSA key

A RSA key can be used both for encryption and for signing and generating a key is quite easy, all you have to do is the following:

  openssl genrsa -des3 -out privkey.pem 2048

That will generate a private key with is password protected (it will prompt you for the password during generation). If you don’t want it password protected (usually for server side use) then leave the -des3 parameter out, i.e.:

  openssl genrsa -out privkey.pem 2048
 The number 2048 is the size of the key, in bits. Today, 2048 or higher is recommended for RSA keys, as fewer amount of bits is considered insecure.

Generating a DSA key

A DSA key can be used for signing only. This is important to keep in mind to know what kind of purposes a certificate request with a DSA key can really be used for.

Generating a key for the DSA algorithm is a two-step process. First, you have to generate parameters from which to generate the key then to generate the key itself.

  openssl dsaparam -out dsaparam.pem 2048
  openssl gendsa -des3 -out privkey.pem dsaparam.pem

Again like RSA, 2048 is the size of the key, in bits with anything smaller than 2048 being insecure in todays standards.

Also the -des3 parameter will prompt you for a pass phrase – for server use leave it out:

  openssl dsaparam -out dsaparam.pem 2048
  openssl gendsa -out privkey.pem dsaparam.pem