WordPress Custom Taxonomy filters

I am needing to create a custom taxonomy structure for my posts. I have read the WP Codex and am confident I can pull that off. My custom taxonomy will be hierarchical.

What I need to do is to create a filter based on this new taxonomy. I need to do this with drop down boxes.

Here is an example of my custom taxonomy will be:

• Artlcles
  • Article 1
  • Article 2
  • Article 3
• Amendments
  • Amendment 1
  • Amendment 2
  • Amendment 3

What I need is to have 2 drop down boxes. The first one will have all of the top level categories (Articles, Amendments, etc…) and then when one of those is selected it will populate/make appear a second drop down with all of the subcategories.

How can I achieve this?

It would not let me post this much code so I have to edit my original question….

This code below is what I have which is on this page http://www.constitutingamerica.org/dev2

<?php $args = array(
    'show_option_all'    => '',
    'show_option_none'   => '',
    'orderby'            => 'ID', 
    'order'              => 'ASC',
    'show_count'         => 1,
    'hide_empty'         => 0, 
    'child_of'           => 0,
    'parent'             => 0,
    'exclude'            => '',
    'echo'               => 1,
    'selected'           => 0,
    'hierarchical'       => 0, 
    'name'               => 'cat',
    'id'                 => '',
    'class'              => 'postform',
    'depth'              => 1,
    'tab_index'          => 0,
    'taxonomy'           => 'classification',
    'hide_if_empty'      => false,
    'walker'             => ''
);

$tax_menu_items = get_categories( $args ); ?>

<form name="class-filter" method="get" >
<select name="class-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
<option value="">Select</option>
<? foreach ( $tax_menu_items as $tax_menu_item ): ?>
    <option value="<?php echo get_term_link($tax_menu_item,$tax_menu_item->taxonomy); ?>"><?php echo $tax_menu_item->name; ?></option>
<?php endforeach; ?>
</select>

<?php
$sub_tax_id = $wp_query->get_queried_object_id();
if(isset($sub_tax_id)) {
$args2 = array(
    'show_option_all'    => '',
    'show_option_none'   => '',
    'orderby'            => 'ID', 
    'order'              => 'ASC',
    'show_count'         => 1,
    'hide_empty'         => 0, 
    'child_of'           => 0,
    'parent'             => $sub_tax_id,
    'exclude'            => '',
    'echo'               => 1,
    'selected'           => 0,
    'hierarchical'       => 0, 
    'name'               => 'cat',
    'id'                 => '',
    'class'              => 'postform',
    'depth'              => 1,
    'tab_index'          => 0,
    'taxonomy'           => 'classification',
    'hide_if_empty'      => false,
    'walker'             => ''
);

$tax_menu_items2 = get_categories( $args2 ); ?>

<select name="class-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
<option value="">Select</option>
<? foreach ( $tax_menu_items2 as $tax_menu_item2 ): ?>
    <option value="<?php echo get_term_link($tax_menu_item2,$tax_menu_item2->taxonomy); ?>"><?php echo $tax_menu_item2->name; ?></option>
<?php endforeach; ?>
</select>

<? } ?>
</form>

I am so close. MY problem is that when the second box is selected it resets the second box to the same parent taxonomy as the first box. How can I get this second drop down to hold all of its original child taxonomies?