South Africa's estimates for key crops such as maize have become increasingly accurate thanks to satellite imagery and as farmers' often biased input has been cut out of the picture, a conference was told on Thursday.

South Africa's maize crop has been hard hit this season by a scorching drought, bringing into sharp focus the need for accurate forecasts of the harvest's size to guide government policy and markets.

From 1997 to 2002, all of the maize forecasts made by the official Crop Estimates Committee (CEC) underestimated the size of the harvest, said Eugene du Preez, director of privately-held SiQ, which provides the committee with satellite and aerial data, which helps it determine the size of the area planted.

"That was a red flag," Du Preez said. The reasons for the underestimates were clear -- the CEC was relying on farmers for much of its information, and they had a transparent incentive to say they had planted less than they had because that would support prices.

Du Preez said from 2003 until 2008, five of the six forecasts underrated the size of the crop.

"We started providing the government with data in 2002, and at that stage we were still very much providing them with information coming from farmers, and there was problems with that," Du Preez told Reuters on the sidelines of a conference on monitoring food insecurity threats from space.

"And then we decided we needed to change the system. So we developed the producer independent crop estimate system."

The system includes satellite images and also surveys conducted with the use of low-flying aircraft.

For yield estimates, the CEC relies in large part on on-site surveys conducted by the state-run ARC Grain Crops Institute.

"They are doing work in the three main maize provinces, Mpumalanga, Free State and North West," said Marda Scheepers, a senior statistician with the CEC.

"They go into a field and they do crop counts and plant counts to get an average yield for the province," she said.

Removing the farmers' input removes the bias.

From 2009 to 2015, the CEC had three underestimates and four overestimates for the size of the crop. And it has been getting closer to the actual crop size.

Du Preez said from 1997 to 2002, its forecasts were routinely 6 or 7 percent off target and even close to 13 percent out. But he said the average since 2009 was 2.6 percent and last year the difference between the CEC's final forecast and the size of the harvest was 0.2 percent.

South Africa will likely harvest 7.1 million tonnes of maize in 2016, 29 percent less than the 9.95 million tonnes reaped last year because of drought and late plantings, the CEC said last month.

Its next forecast is due next week and a Reuters poll sees it cutting its last estimate by 5.5 percent to 6.7 million tonnes.