Bu makale, Tuz Gölü’ndeki suyla kaplı alanlarda meydana gelen zamansal değişimin eş-zamanlı uzaktan algılama verileri ile analiz edilmesini konu almakta olup iki aşamada gerçekleştirilmiştir: (1) Eş-zamanlı yer ve uydu verilerinin birlikte analizi (2) Çok zamanlı LANDSAT uydu verileri ile zamansal değişim analizi. Çalışmanın ilk aşamasında, Tuz Gölü ve yakın çevresinde ayrıntılı bir arazi çalışması yapılmış ve LANDSAT-5 uydusunun 16.05.2005 (saat:1031) tarihindeki üst geçişiyle eş-zamanlı olarak ve yer koordinatları el tipi GPS ile belirlenen noktalarda yersel spektroradyometre ölçmeleri gerçekleştirilmiştir. Uydu görüntülerinin işlenmesi aşamasında geometrik ve radyometrik düzeltme işlemleri gerçekleştirilmiştir. Geometrik düzeltme işlemi ile tüm uydu görüntüleri UTM (Universal Transverse Mercator) projeksiyon sistemine (36. Dilim) referanslandırılmıştır. Bu işlem esnasında karesel ortalama hata (KOH) tüm uydu verileri için ±0,5 pikselin altında bulunmuştur. Daha sonraki aşamada ise uydu verilerini yersel spektroradyometre ölçmeleri ile karşılaştırılabilir hale getirmek amacıyla radyometrik düzeltme işlemi gerçekleştirilmiştir. Bu aşamada, LANDSAT-5 TM uydu verisine ait parlaklık değerleri öncelikle radyans, daha sonra reflentans değerlerine dönüştürülmüştür. Radyometrik düzeltme işlemi sonrasında LANDSAT-5 TM uydu verisinden ve eş-zamanlı yersel spektroradyometre ölçmeleriyle elde edilen yansıma değerlerinin yüksek korelasyona (0.84<R2<0.97) sahip oldukları belirlenmiştir. Daha sonraki aşamada, çok zamanlı LANDSAT-5 uydu verileri ile zamana bağlı değişim analizi gerçekleştirilmiş ve Tuz Gölü’ndeki suyla kaplı alanların 1990-2005 yılları arasında 1/3 oranında azaldığı tespit edilmiştir. Göl çevresinde su kaynaklarının kullanımının kontrol altına alınması ve gölün (en az yılda bir kez olmak üzere) güncel uzaktan algılama verileri ile düzenli olarak izlenmesi önerilmektedir. Anahtar Kelimeler: Tuz Gölü, uzaktan algılama, spektroradyometre, radyometrik düzeltme.This paper focuses mainly on the investigation of water reserve changes in Salt Lake, Turkey, using real-time and multitemporal remote-sensing data. The Salt Lake, which is a specially protected area, is natural resource and the second largest lake in the Central Anatolia in Turkey. The region has been experiencing drought over the last two decades resulting from two main phenomena; (1) uncontrolled use of underground water resources for agricultural purposes, and (2) the lack of precipitation (or natural drought). As a result of this event, very striking coastline changes occurred in the Salt Lake and its vicinity within the last 20-year period. In this study, these changes were analyzed by using multitemporal LANDSAT-5 imagery and real-time ground spectral measurements carried out around Salt Lake, Turkey. The study is performed in two stages: (1) correlation analysis for real-time ground spectral measurements and LANDSAT-5 TM image data and (2) assessment of water reserve changes using multi-temporal LANDSAT imagery. In the first stage of the study, the relationship between ground and satellite spectral data captured on the same day and time was investigated. For this purpose, a real-time field work was done in May, 2005. Taking into account the overpass of LANDSAT-5 TM (16 May 2005, 10:31 local time), spectral measurements were taken on the same day between 9:30 and 12:30 in a total of 20 sample points along the northern coasts of Salt Lake. Sample points were selected along four paralel transects representing four major cover types naturally and regularly located along the coast: lake water, salt, muddy area and bare soil. The distance between samples was approximately 300 m and their locations were recorded with a hand-held global positioning system (GPS) receiver.The measurements were done using an ASD FieldSpec®Pro field-portable spectroradiometer, which consists of a data analyser attached to a laptop computer. In the next stage, LANDSAT scenes (for 1990, 2001 and 2005) covering the study site were geometrically corrected. The images were converted to the UTM coordinate system (Zone 36) using a total of 50 control points both extracted from 1:25000-scale topographic maps and recorded by GPS during the fieldwork. A first order polynomial transformation method was performed to create the output images with 30 m ground resolution. The root-mean-square error of the polynomial transformation is less than half a pixel for all the data set. In the radiometric correction procedure, first, the LANDSAT-5 TM 2005 image and then the 1990 and 2001 historical images were radiometrically corrected to enable comparison with in-situ measurements and with each other. The aim of radiometric correction is to minimize the atmospheric effects and to convert remotely sensed digital numbers (DNs) to ground surface reflectance in order to make the data spectrally comparable. The procedure includes conversions from digital numbers to radiance and from radiance to at-satellite reflectance. After the radiometric correction procedure, the correlation between real-time ground and satellite data was examined. The regression results show that measured surface reflectance and converted at-satellite reflectance are in good agreement with R2 values of about 0.95 in the selected study area. The results show that simultaneous ground and LANDSAT-5 TM data are highly correlated (0.84 R2 0.97). Here, it can be indicated that the use of the real-time data set collected on the same day and time increased the consistency between ground and satellite data. The results also show that sensing using just the spectral band in the near infrared region, such as by LANDSAT TM4, is favourable for distinguishing salt and water in Salt Lake, which has a salt-covered bottom. This plays a key role in determining multi-temporal water reserve changes using satellite sensor data.The results of multi-temporal analysis show that the water reserves in Salt Lake, Turkey, have decreased markedly over two decades. The change of water reserve quantified by using LANDSAT images from 1990 and 2005 is more than 43 ha for a 15-year period. It is suggested that the use of water supplies around Salt Lake should be controlled and that the lake should be monitored regularly by current remote-sensing data (at least annually) for better management of the water resources. Keywords: Salt Lake, remote sensing, field spectroradiometer; radiometric correction.
