1 Introduction

The origin of the neutral-hydrogen HVCs has been an enigma for the last 36 years. The main problem in the study of these objects are the distances: they are unknown for most of the HVCs and, therefore, few of the basic physical properties of the clouds can be determined. For a review of problems and progress, see Wakker & van Woerden ([1997]).

Figure 1: Sky distribution of the 6848 HV components detected from the IAR H I survey with velocities $V_{LSR} \geq 80$ km s-1. Black and grey dots are for positive and negative velocities, respectively

The northern sky has been surveyed several times looking for HVCs (see Table 1 in the review of Wakker & van Woerden [1997]). Recently, Hartmann & Burton ([1997]) finished the Leiden-Dwingeloo HI survey made with a finer grid and higher velocity resolution than previous ones. This survey can be used for the creation of a catalogue for all the anomalous-velocity gas. However, for the southern sky the investigation of HVCs was restricted to the use of old surveys. It was first surveyed for HVCs by Mathewson et al. [1974]), using the 18-m reflector at the Parkes Observatory of CSIRO (HPBW $\sim$ 48$^\prime$), every 2.5 to 5 degrees in declination and moderate sensitivity (typically 0.3 K). The main result of this survey was the finding of the extended HI complex known as the Magellanic Stream. Later, Bajaja et al. ([1985]) made a new survey using the 30-m IAR dish (HPBW $\sim$ 34$^\prime$), on a 2$^\circ$$\times$ 2$^\circ$grid and a detection limit of 0.08-0.10 K. As a result of these observations, a large number of small clouds was discovered. As we can see, a drawback of these surveys was the fact that the antenna beams were smaller than the observed grid spacing. More recently, Putman & Gibson ([1999a], [1999b]) presented the first results from their complete, fully sampled, southern sky survey for HVCs, as part of the HI Parkes All-Sky Survey ( ). As they mentioned, their initial results are revealing intricate details in the spatial structure of southern HVCs, a large population of compact HVCs, and filamentary connections between previously assumed discrete clumps. Thus, until these data become of public domain, the data available at present on HVCs are quite insufficient for statistical purposes.

We decided to use the new general HI survey from the southern hemisphere, recently completed by Arnal et al. ([2000]), to make a more complete and homogeneous new survey of HVCs which will provide reliable and unbiased statistical information on these enigmatic objects.

Figure 2: LSR velocity-longitude diagrams from the IAR data for a) the northern and b) southern hemisphere

Figure 3: a) LSR and b) GSR velocity-longitude diagrams from the IAR and Hulsbosch & Wakker's ([1988]) data