AFAIK nginx alias goes like this:
alias <the base path>
means that with it you tell the nginx the part to the left of your first "/" in the URI.
example. suppose I have my images in /var/www/html/images and my application refers to apic.jpg as http://app.com/pictures/apic.jpg , what I do then is this:
location ^/pictures {
alias /var/www/html/images;
}
hope this helps.