Atrial repolarization wave (Ta wave) is seldom used as an indicator of atrial arrhythmia due to the lack of perceiving it in standard 12-lead Electrocardiogram (ECG), where the QRS complex obscures the low-voltage Ta wave. Hence, this study proposes spline model to interpolate the hidden Ta wave within the QRS complex using the data points from the PR and ST segments. Fifty Sinus Rhythm (SR) and fifty Sinus Tachycardia (ST) 10 s denoised ECGs were taken from the Chapman University and Shaoxing People's Hospital database. Each ECG was segmented into single-beat ECGs after creating the window surrounding the R peak and was further averaged. PR and ST segment data points from averaged ECG were taken and an arc interpolation was plotted in ST. P and Ta wave characteristics were acquired and further analyzed. Ta peak location lies within the QRS complex however near the PR segment in ST condition. This study also introduces a new segment in SR called QRS inter-segment duration, i.e., end of the P wave to beginning of the T wave, which corresponds to the Ta wave duration in ST as the Ta wave does not manifest in SR. This segment will help to understand the probable Ta wave duration variations in SR. Moreover, through the correlation plot between the Ta and P wave with HR, it is noted that Ta correlates better (r = 0.5). Age-wise alterations in Ta wave were examined and found that Ta wave duration and amplitude increase with the age in SR and ST; however, the increase in the Ta wave duration with age in ST condition is not as frequent to that of the SR condition. To conclude, Ta wave is interpolated in ST condition and its features were extracted to analyze the wave variations between SR and ST. Further insightful studies are needed to study Ta wave characteristics in different heart conditions.