Striping pada Citra Landsat 7
Jika anda memiliki citra Landsat 7 ETM+ pasti tidaklah asing dengan masalah striping (garis-garis/strip tanpa informasi). Striping sendiri disebabkan oleh Scan Line Corrector (SLC) pada satelit Landsat 7 yang mengalami masalah pada tanggal 31 Mei 2003 dan tidak bisa diperbaiki sehingga mengakibatkan kerusakan permanen pada citra yang diakuisisi setelahnya. Tanpa beroperasinya SLC, sensor satelit memindai area pencitraan dalam pola zig-zag seperti pada ilustrasi berikut:
Alhasil, citra menjadi terduplikasi terutama pada bagian tepi scene. Ketika data Level-1 diproses, area yang terduplikasi ini akan dihapus secara otomatis namun akan menghasilkan ‘gap‘. Sehingga dalam satu scene hanya terdapat 78% pixel area saja yang memiliki informasi, sisanya (22%) menjadi null values yang nampak menjadi garis strip tanpa data.
Tools yang Dibutuhkan dan Prosesnya
Dalam tulisan ini saya akan membagikan cara untuk ‘mengisi’ area striping tersebut, sehingga citra Landsat 7 ETM+ bisa dimanfaatkan dengan baik. Adapun tools yang dibutuhkan kali ini semuanya opensource, yaitu QGIS dan Semi Automatic Classification Plugin (SCP). Untuk QGIS sendiri bisa diunduh di sini, dan untuk SCP bisa diunduh di sini.
SCP sendiri bisa di-install melalui antarmuka QGIS dengan cara berikut, namun tahap ini bisa dilewati apabila plugin SCP telah ter-install sebelumnya.
- Pilih menu Plugins > Manage and Install Plugins..
- Pada jendela Plugins pilih Install from ZIP, lalu browse ke lokasi file *.zip SCP yang sebelumnya telah diunduh dengan menekan tombol […].
- Tekan Install Plugin, dan tunggu hingga proses instalasi selesai. SCP yang sudah berhasil di-install akan muncul di menu-bar QGIS.
Terdapat 2 proses utama dalam proses destriping, yaitu (1) Preprocessing citra dan (2)Destriping.
1. Preprocessing Citra
Tahapan ini dilakukan untuk mengkonversi citra Landsat 7 yang masih dalam Digital Number (DN) ke nilai reflektansi TOA (Top of Atmosphere) dan sekaligus dilakukan koreksi atmosferik sederhana dengan metode DOS1. Adapun tahapannya adalah sebagai berikut:
- Pilih menu SCP > Preprocessing > Landsat
- (1) Browse direktori hasil ekstraksi data Landsat 7. (2) Browse file *MTL.txt dari direktori Landat 7. (3) Checklist opsi Apply DOS1 atmospheric correction. (4) Uncheck opsi only to blue and green bands. (5)Tekan RUN dan kemudian tentukan direktori penyimpanan file hasil preprocessing, lalu pilih Select folder.
- Setelah proses selesai maka citra hasil preprocessing akan secara otomatis termuat di dalam lembar kerja QGIS.
2. Destriping
Pada tahap ini dilakukan proses penghilangan area strip (destripe) dengan cara mengisi area strip dengan interpolasi dari area disekitarnya. Proses ini membutuhkan guidence dari layer mask, sehingga software dapat mengetahui di mana saja area yang perlu di-interpolasi. Untungnya USGS sudah mem-bundle layer mask ini untuk masing-masing band yang dapat anda temukan datanya di folder gap_mask pada direktori files Landsat 7. Layer mask ini didistribusikan dalam format GeoTIFF (*.tif) yang dikompres dalam file *.zip, sehingga harus didekompresi dulu semuanya.
Adapun proses destripe ini dilakukan dengan tahap-tahap berikut ini:
- Pilih menu Raster > Analysis > Fill nodata dari jendela QGIS.
- Pada parameter Input layer tentukan layer mana yang akan di destripe.
- Pada parameter Maximum distance (in pixel) to search out for values to interpolate masukkan nilai 50 (nilai ini merupakan radius pencarian nilai sekitar untuk proses interpolasi, anda pun bebas untuk bereksperimen untuk memasukkan angka lain pada parameter ini).
- Pada parameter Validity mask [optional], pilih layer mask yang sesuai dengan band yang bersangkutan, jika tidak sesuai maka proses destripe tidak akan berhasil (contoh: jika Input layer = RT_LE07_L1TP_XXXXXX_XXXXXXXX_XXXXXXXX_01_T1_B1, maka layer mask-nya harus = LE07_L1TP_XXXXXX_XXXXXXXX_XXXXXXXX_01_T1_GM_B1).
- Pada parameter Filled tentukan di path mana file hasil destripe akan disimpan (atau bisa menambahkan ekstensi _destripe, untuk mempermudah identifikasi), kemudian pilih Run untuk mengeksekusi perintah.
- Setelah proses selesai maka citra hasil destripe akan termuat otomatis di dalam QGIS dan pada area strip sudah terisi data sebagaimana berikut:
- Lakukan proses yang sama (Fill nodata) pada band 1,2,3,4,5, dan 7.
3. Stacking Band (Opsional)
Tahapan ini dilakukan untuk ‘menumpuk’ (stacking) band-band yang masih terpisah dalam file yang berbeda menjadi dalam satu ‘wadah’. Proses ini sendiri bisa dilakukan via SCP melalui modul Band set, dengan tahapan sebagai berikut:
- Buka menu SCP > Band Set
- Dari jendela SCP yang muncul, (1) Buka list citra hasil destriping. (2) List tersebut akan masuk ke Single band list termasuk untuk citra yang belum dilakukan destriping. (3) Beralih ke bagian Band Set 1, pastikan band destripe sudah masuk ke dalam list ini, dan untuk menambahkan band bisa menggunakan tombol [+] dan untuk mengeliminasi dengan menggunakan tombol [-], selain itu pastikan urutannya sudah benar yaitu 1,2,3,4,5,7, untuk mengatur urutannya bisa digunakan tombol panah atas [↑] dan panah bawah [↓] yang ada di task pane sebelah kanan.
- Pilih Landsat 7 ETM+ [bands 1, 2, 3, 4, ,5, 7] dari list Quick wavelength settings
- Berikan checklist pada opsi Create raster of band set…
- Klik tombol RUN, maka akan muncul jendela yang meminta lokasi penyimpanan, dan pilih Select Folder jika sudah selesai.
Hasil dari proses destriping pada citra Landsat 7 ETM+ dengan menggunakan QGIS cukup baik, simpel, dan tentunya gratis, dan berikut ini contoh dari citra Landsat 7 ETM+ di daerah Kalimantan Timur sebelum dan sesudah proses destriping yang dilakukan dengan proses di atas.
Sumber: https://www.usgs.gov/land-resources/nli/landsat/landsat-7?qt-science_support_page_related_con=0#qt-science_support_page_related_con https://www.usgs.gov/faqs/what-landsat-7-etm-slc-data?qt-news_science_products=0#qt-news_science_products http://dx.doi.org/10.13140/RG.2.2.29474.02242/1 https://docs.qgis.org/2.8/en/docs/user_manual/processing_algs/gdalogr/gdal_analysis/fillnodata.html
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