  (Shortened version) Current efforts in space weather forecasting of CMEs have
been focused on predicting their arrival time and magnetic structure. To make
predictions, methods have been developed to derive the true CME speed, size and
position, among others. Difficulties in determining input parameters for CME
forecasting arise from the lack of direct measurements of the coronal magnetic
fields and uncertainties in estimating the CME 3D geometric and kinematic
parameters. White-light coronagraph images are usually employed by a variety of
CME reconstruction techniques. We explore how subjectivity affects the 3D CME
parameters that are obtained from the GCS reconstruction technique. We have
designed two different synthetic scenarios where the ``true'' geometric
parameters are known in order to quantify such uncertainties for the first
time. We explore this as follows: 1) Using the ray-tracing option from the GCS
model software, and 2) Using 3D MHD simulation data from the MAS code. Our
experiment includes different viewing configurations using single and multiple
viewpoints. CME reconstructions using a single viewpoint had the largest errors
and error ranges for both synthetic GCS and simulated MHD white-light data.
Increasing the number of viewpoints to two, the errors decreased by about
4$^\circ$ in latitude, 22$^\circ$ in longitude, 14$^\circ$ in tilt, and
10$^\circ$ in half-angle, pointing towards a need for at least two viewpoints.
We found the following CME parameter error bars as a starting point for
quantifying the minimum error in CME parameters from white-light
reconstructions: $\Delta\theta$ (latitude)=${6^\circ}$, $\Delta\phi$
(longitude)=${11^\circ}$, $\Delta\gamma$ (tilt)=${25^\circ}$, $\Delta\alpha$
(half-angle)=${10^\circ}$, $\Delta h$ (height)=$0.6 R_{\odot}$, and
$\Delta\kappa$ (ratio)=$0.1$.
