wmsmap2scaledenominator

#!/usr/bin/env bash
PROGRAM_NAME=$(basename "$0")
URL="$1"

function usage {
    echo "Calculate the scale denominator of a WMS GetMap request."
    echo "When no DPI parameter provided in the URL (e.g. \`DPI=92\`), "
    echo "the script assumes a default DPI value of 90.71446714322."    
    echo ""
    echo "usage: $PROGRAM_NAME <getmap_url> "
    echo "    - <getmap_url>: WMS GetMap request URL"
    exit 1
}

# see https://stackoverflow.com/a/37840948
function urldecode() { 
    : "${*//+/ }" 
    echo -e "${_//%/\\x}"
}

if test "$#" -ne 1; then
    usage
fi

# shellcheck disable=SC1001
if ! [[ $URL = \http://*  || $URL = \https://* ]];then
    usage
fi

set -euo pipefail

URL="$(urldecode "$URL")"
QUERY_STRING=$(cut -d? -f2 <<<"$URL")
declare -A param

while IFS='=' read -r -d '&' key value && [[ -n "$key" ]]; do
    param["$key"]=$value
done <<<"${QUERY_STRING}&"

BBOX=${param[BBOX]}
IMAGE_WIDTH=${param[WIDTH]}
MIN_X=$(cut -d, -f1 <<<"$BBOX")
MAX_X=$(cut -d, -f3 <<<"$BBOX")
DELTA_X=$(bc <<<"$MAX_X-$MIN_X")

if [ ${param[DPI]+_} ]; then
    DPI=${param[DPI]}
else
    echo "NO DPI query param found, using default value for DPI: 90.71446714322"
    DPI="90.71446714322"
fi

# SCALE: "1:${SCALEDENOMINATOR}"
# SCALEDENOMINATOR = $REALWORLD_DISTANCE/$SCREEN_DISTANCE
SCREEN_DISTANCE=$(bc -l <<<"(((${IMAGE_WIDTH}/${DPI})*2.54)/100)")
SCALEDENOMINATOR=$(bc -l <<<"${DELTA_X}/${SCREEN_DISTANCE}")
echo "${SCALEDENOMINATOR}" | xargs printf "%.2f"