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111 lines
2.7 KiB
Markdown
111 lines
2.7 KiB
Markdown
---
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title: "Retreiving GPS data in decimal format from EXIF data in photos"
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date: 2022-06-19T19:01:35-04:00
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draft: false
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tags: ["GPS"]
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math: false
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---
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For a new feature that I'm cooking up for my website, I need to grab the GPS information from the EXIF data stored in my images. Luckily, `imagemagick`
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comes to our rescue.
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```bash
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identify -verbose $IMAGE_FILE | grep GPS
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```
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This will return something like the following:
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```
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exif:GPSAltitude: 1250/100
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exif:GPSAltitudeRef: .
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exif:GPSDateStamp: 2022:06:12
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exif:GPSImgDirection: 137/1
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exif:GPSImgDirectionRef: M
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exif:GPSInfo: 1004
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exif:GPSLatitude: 40/1, 50/1, 1815/100
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exif:GPSLatitudeRef: N
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exif:GPSLongitude: 73/1, 53/1, 3625/100
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exif:GPSLongitudeRef: W
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exif:GPSTimeStamp: 17/1, 32/1, 30/1
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exif:GPSVersionID: ....
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```
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To request a specific field, for example Latitude:
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```bash
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identify -format "%[EXIF:GPSLatitude]\n" "$IMAGE_FILE"
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```
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As with the verbose flag, it will return the information in degrees format
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```
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40/1, 50/1, 1815/100
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```
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The following bash function will take the degrees format and convert
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it to the more common decmial format:
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```bash
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DegreesToDecimal() {
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L0=$(echo "$1" | cut -d "," -f 1)
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L1=$(echo "$1" | cut -d "," -f 2)
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L2=$(echo "$1" | cut -d "," -f 3)
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echo "scale=6;$L0 + $L1/60 + $L2/3600" | bc
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}
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```
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For example:
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```bash
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LAT=$(identify -format "%[EXIF:GPSLatitude]\n" "$IMAGE_FILE")
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LAT_DEC=$(DegreesToDecimal "$LAT")
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echo "$LAT_DEC"
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```
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will output:
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```
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40.838374
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```
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We can then package this into a script which will output the latitude, longitude, and altitude (m) of an image in decmial format.
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```bash
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#!/bin/sh
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set -o errexit
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set -o nounset
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set -o pipefail
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show_usage() {
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echo "Usage: getLLA.sh [imagefile]"
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exit 1
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}
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# Check argument count
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if [ "$#" -ne 1 ]; then
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show_usage
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fi
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# Check that relevant command exist
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if ! command -v identify > /dev/null; then
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echo "Command identify from imagemagick not found. Exiting..."
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fi
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IMAGE_FILE="$1"
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LAT=$(identify -format "%[EXIF:GPSLatitude]\n" "$IMAGE_FILE")
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LAT_DIR=$(identify -format "%[EXIF:GPSLatitudeRef]\n" "$IMAGE_FILE")
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LON=$(identify -format "%[EXIF:GPSLongitude]\n" "$IMAGE_FILE")
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LON_DIR=$(identify -format "%[EXIF:GPSLongitudeRef]\n" "$IMAGE_FILE")
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ALT=$(identify -format "%[EXIF:GPSAltitude]\n" "$IMAGE_FILE")
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DegreesToDecimal() {
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L0=$(echo "$1" | cut -d "," -f 1)
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L1=$(echo "$1" | cut -d "," -f 2)
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L2=$(echo "$1" | cut -d "," -f 3)
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echo "scale=6;$L0 + $L1/60 + $L2/3600" | bc
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}
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LAT_DEC=$(DegreesToDecimal "$LAT")
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LON_DEC=$(DegreesToDecimal "$LON")
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ALT_DEC=$(echo "scale=6;$ALT" | bc)
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LAT_PREFIX=$([ $LAT_DIR == "S" ] && echo "-" || echo "")
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LON_PREFIX=$([ $LON_DIR == "W" ] && echo "-" || echo "")
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echo "$LAT_PREFIX$LAT_DEC"
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echo "$LON_PREFIX$LON_DEC"
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echo "$ALT_DEC"
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```
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