Heat oven to 350 degrees F. Grease and flour 10-inch (12 cup) tube pan.
Sift together flour, baking powder, nutmeg, cinnamon, allspice and salt into large bowl.
Mix together buttermilk and vanilla in small bowl.
Beat butter in bowl at medium speed until smooth and creamy. Beat in brown sugar until fluffy, 2 minutes. Beat in eggs, one at a time. At low speed, beat in flour mixture in 3 additions alternately with buttermilk mixture. Fold in chopped pecans. Pour into tube pan. Arrange pecan halves on top. Cover loosely with foil.
Bake in 350 degree F oven for 1 hour 20 minutes. Remove foil. Bake for 20 minutes, until pick inserted in crack on top comes out with moist crumbs attached. Cool cake in pan on wire rack 20 minutes.
Bring sugar and water to boil in small saucepan until sugar is dissolved, about 1 minute. Lift cake out of pan. Place waxed paper under rack; place cake on rack. Brush cake with glaze. Cool completely. (Cake can be made several weeks ahead; wrap in plastic wrap, then foil; freeze. Thaw overnight in refrigerator.) Makes 24 servings.