My approach to this, which may not be the simplest, will still get you where you need to be. If you absolutely need to have letters for unique identified, I would create a CSV with two columns: ID and LetterID. ID will be numbers 1-500 and LetterID will be a two character string. There are 676 possible letter combinations with two characters and 26 letters of the alphabet.
To create the list of letters, write a simple python script with embedded for loops. This is a quick, lazy one I just wrote:
first = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']
second = ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z']
f = open ('UniqueLetter.txt', 'w')
for a in first:
for b in second:
f.write(a+b+ '\n')
f.close()
Copy the values of your text doc into excel, crop to 500 values and add your join field (1-500). Save as CSV, import to QGIS, join to your shapefile and save or use field calculator to copy the unique letter combinations into a new field.
CVE_ENT
to character type? You can even make it overtly alphanumeric by, say, prepending some common prefix to it, such as converting28
to"ID28"
, etc. That should satisfy any software that insists the key be alphanumeric.